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Design and fabrication of reliable electrodes is one of the most important challenges in flexible devices, which undergo repeated deformation. In conventional approaches, mechanical and electrical properties of continuous metal films degrade gradually because of the fatigue damage. The designed incorporation of nanoholes into Cu electrodes can enhance the reliability. In this study, the electrode shows extremely low electrical resistance change during bending fatigue because the nanoholes suppress crack initiation by preventing protrusion formation and damage propagation by crack tip blunting. This concept provides a key guideline for developing fatigue-free flexible electrodes.

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gravimetric and volumetric capacity, which is much larger than that of any other commercialized anode material. [ 9,10 ] Bulk Si itself is extremely brittle when it is subject to bending conditions so an ultrathin Si fi lm or ribbon has been introduced to fabricate Si-based fl exible devices. [11][12][13][14][15][16] However, ultrathin Si is still insuffi cient for stable cycling of bendable batteries because Si anodes should accommodate the large volume expansion of Si during charge/discharge cycles. Si exhibits severe mechanical degradation (e.g., cracks, delamination) due to the accumulated stress caused by large volume expansion, which can be as high as 300%, upon full lithiation, [ 17,18 ] thus resulting in a fatal capacity decay and a short cycle life. [ 19,20 ] Serious reliability issues, such as the acute delamination of the Si thin-fi lm anodes on polymer substrates, could be even more severe compared with Si thin fi lms on metal (e.g., Cu) current collector. Therefore, if we introduce a new design approach to create highly reliable nanostructured electrodes that are necessary for the implementation of Si anodes on PI substrates without sacrifi cing battery performance, our direct implementation scheme can be used for highly reliable nanostructured electrodes that are necessary for polymer-based fully bendable and foldable devices. In an attempt to overcome the mechanical instability of Si, various Si nanostructures, such as simple nanowires (NWs), core/shell NWs, coated NWs, embedded NWs and nanotube NWs, [21][22][23][24][25][26] based on Si NWs have been explored, resulting in strain release from free expansion and fracture resistance through critical size effects. [27][28][29][30] However, designing these nanostructured Si anodes on PI substrates is extremely challenging due to diffi culties in the direct integration onto the polymer substrates compared with the metal substrates. For instance, Si NWs were normally obtained by vapor-liquid-solid (VLS), supercritical fl uid-liquid-solid (SFLS) or solution-liquid-solid (SLS) growth synthesized at temperatures approximately between 300 and 1000 °C, [31][32][33] which is too high to use the polymer-based substrates. Here, we fabricate a nano-hairy Si electrode directly onto the PI substrate. By implementing an in situ lithiation test, we demonstrate the lithiation mechanism of our unique nano-hairy structure, which facilitates the kinetic lithiation process and the strain accommodation process. Furthermore, using a button-cell battery test and a bendable pouch cell battery test, we demonstrate that our new electrode design signifi cantly improved the cycling and rate performance of the battery, as well as successfully overcome the aforementioned fatal issues of the Si anode integrated onto a polymer substrate. We believe that our direct integration scheme using a nano-hairy structure is applicable not only to Si but also to general electrode materials for polymer substrate-based bendable Li-ion batteries, and this strategy can facilitate a new application ...