We have produced stretchable lithium-ion batteries (LIBs) using the concept of kirigami, i.e., a combination of folding and cutting. The designated kirigami patterns have been discovered and implemented to achieve great stretchability (over 150%) to LIBs that are produced by standardized battery manufacturing. It is shown that fracture due to cutting and folding is suppressed by plastic rolling, which provides kirigami LIBs excellent electrochemical and mechanical characteristics. The kirigami LIBs have demonstrated the capability to be integrated and power a smart watch, which may disruptively impact the field of wearable electronics by offering extra physical and functionality design spaces.

There is a great deal of interest in developing next-generation lithium ion (Li-ion) batteries with higher energy capacity and longer cycle life for a diverse range of applications such as portable electronic devices, satellites, and next-generation electric vehicles. Silicon (Si) is an attractive anode material that is being closely scrutinized for use in Li-ion batteries because of its highest-known theoretical charge capacity of 4200 mAh g −1 .[1] The development of Si-anode Li-ion batteries has been hindered, however, mostly because of the large volumetric changes (up to 400%) that occur upon insertion and extraction of Li ions, and in turn the large electrochemically related stress, which results in electrode pulverization, loss of electrical contact, and early capacity fading of battery cells. [2][3][4][5] Despite this challenge, the extraordinarily high energy capacity of Si in its own right has motivated researchers to develop new techniques that reduce the limitations of Si as a practical anode material. Ultrathin Si films down to 50 nm in thickness have been reported for successful antipulverization and capacity nondegradation over two thousand charge/discharge cycles on roughened current collectors. [6] This result, together with a surge of work on improving the capacity retention of Si anodes such as nanoparticles [7,8] and/or composites, [9][10][11][12] nanowires, [13][14][15] or nanotubes [16,17] have shown improved performances, where the nanoforms of materials can offer expansion spaces during lithium insertion/extraction ( Figure 1A). However, some degree of capacity fading still exists due to the limited space for accommodating the facile strain expansion as well as decreased accessibility of the electrolyte to the solid -electrolyte interphase (SEI) between the silicon nanostructures and electrolyte. Here, we present a new strategy of stress relaxation for Si films using an elastomeric substrate that will establish an alternative route for new electrode design. In addition, the design of the anodes offers more efficient ion and electron transport than the reported work that uses nanoparticles, nanowires, or nanotubes.The general concept of stress relaxation can be understood using an eigen strain analogy. It is well-known that the eigen deformation of a free-standing material does not lead to mechanical stress, but only to self-compatible deformations, and eigen-strain-induced stresses are generated when the eigen strain is constrained. Consequently, the stress can be released by removing these constraints (e.g., stainless steel [13] and rough substrates [6] ). Herein, we report an approach in which the rigid substrates (e.g., current collectors) that constrain the "free" expansion/contraction of the Si anodes during charge/ discharge are replaced by soft substrates. The mechanism for stress relaxation is that the volumetric strain in Si that is induced by charge/discharge cycling can buckle the flat Si thin films when they are on soft substrates ( Figure 1B), which in turn releases the stress ...

Typical bilayer wrinkle systems employ soft elastomers as the substrates. In contrast, shape memory polymers have recently emerged as attractive alternatives. Besides the shape fixing capability, shape memory polymers distinguish from elastomers in that they are rigid at room temperature, but experience significant modulus drop upon heating. We hereby report unique aspects of shape memory polymers as the wrinkle substrate utilizing a metallic thin film as the top layer. The feasibility to create both reversible and irreversible wrinkles (and diffraction colors) on a single substrate is demonstrated. Experimental conditions are identified to create crack free wrinkles and the impact of various experimental parameters on the wrinkle wavelength and amplitude is investigated. The results suggest that the wrinkle mechanics deviate notably from the existing theories established with elastomers as the wrinkle substrates. Thus, a new theory will need to be developed in the future, taking into account of unique thermomechanical properties of the shape memory substrate and possible plastic deformation of the thin film.

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