Preface

Carbon-based materials have continuously drawn significant scientific attention as their adaptability to diverse energy related applications, especially concerning flexible and/or micro energy devices. In this issue, we have collected 7 original research articles and 8 comprehensive reviews focusing on the use of carbon and its related materials for energy devices, including flexible energy storage systems like solid-state Li ion batteries and electrochemical capacitors,Zn ion batteries or hybrid capacitors, K ion batteries and energy harvester like perovskite solar cells. Among these, micro energy devices include micro capacitors, Li-ion micro batteries, Zn micro batteries and 3D printed micro batteries. Carbon-based materials here, play multi-faceted roles as electrode active materials, flexible conductive substrates, carbon coating/buffer layers and porous hosts for active material confinement. These results solidify the legacy of carbon and exemplify the promising applications of carbon-based materials toward flexible and/or micro energy devices.

Concerning flexible energy devices,. reported the design of flexible and integrated rechargeable Li ion batteries constructed from coaxial carbon fiber@FeNiMnOas anode. Such coaxial configuration enables flexibility to a low-strain structure and excellent electrochemical performance benefiting from pre-determined spatial cation occupancy of the FeNiMnO, the avoidance of direct electrolyte contact, and high electrical conductivity. The resulting flexible battery using the as-designed flexible anode against LiFePOcathode in a boron nitride modified polyethylene oxide electrolyte displayed a large areal capacity and decent cycling stability under different bending and strain states.. developed a facile yet efficient strategy to construct superhierarchical carbon networks with nitrogen-doped micro-meso-macroporous carbon network through a Schiff-base gelation and subsequent carbonization. The electrodes applied to a capacitor with high specific capacitance, mechanical flexibility, and long-cycle performance.. proposed the surface oxidation and intercalation modification of hard carbon using ZnClfor enhanced electrochemical performances as anode in lithium-ion capacitors. The capacity and long cycling retention in both half and full cells improved greatly as compared with those of the untreated hard carbon.This was attributed to the presence of oxygen-containing functional groups and widened interlayer spacing, pore size and specific surface area upon modification.

To bring a holistic outlook on the achievements of carbon-based materials regarding flexible energy storage systems, we feature some in-depth review articles covering the material synthesis and device fabrication aspects specific to some emerging technologies. For example,. summarized the recent progress in carbon materials for flexible electrochemical potassium storage devices(K-ion batteries, K-ion hybrid capacitors, and K-S/Se batteries) and discussed various strategies for fabricating carbon-based flexible electrodes and achieving high electrochemical performances in detail.. summarized recent progress in the fabrication of freestanding carbon electrodes towards flexible capacitors. Such freestanding carbon electrodes could become great platforms for building practical flexible capacitors, therefore a perspective about their future and methods to overcome challenges was presented. Beyond K ion-based energy storage, Zn ion batteries and capacitors have gained considerable momentum. The in-depth review article contributed by. elucidates vital role that carbon materials (e.g., carbon nanotubes, carbon fibers, graphene) have played in realizing high-performance flexible Zn ion batteries boasting superior electrochemical performances through various configurations. The challenges and perspective were discussed to further promote the practical application of Zn ion batteries.Among the many virtues of carbon materials, excellent electrical conductivity and mechanical stability of low-dimension carbon allotropes like graphene and carbon nanotubes have shone the brightest. A comprehensive overview mainly centered around the use of these two materials in Zn ion capacitors was provided by. Moreover, in the field of energy storage there are numerous considerations beyond electrochemical performance, and the ability of carbon to cater to these is truly astounding. Along these lines,. summarized the research advances of porous carbon materials applied in different energy storage devices,and proposed strategies to achieve fine structural control for targeted applications.. summarized the self-healing polymer binders to repair the internal and/or external damages of electrodes caused by the huge volume change of Si,which were cross-linked or assembled by noncovalent and reversible covalent bonds during synthesis. Subsequently, they summarized the recent applications of self-healing polymer as solid polymer electrolytes for flexible lithium batteries.. explored the universal chemical crosslinking between the adjacent grains through the interfacial embedding of laser-derived carbon dots with tailored halogen-terminated surfaces to improve the flexibility and stability of polycrystalline perovskite films. Strong interactions between perovskite and carbon dots result in effective defects passivation, lattice anchoring and carrier dynamics modulation of perovskite films.

Scaling down to the level of micro energy devices,. show the planar interdigitated Zn micro batteries by combining CVD, laser patterning and electrodeposition. The highly conductive network and interconnected nanosheet structure facilitate the electron/ion transport, and therefore the Zn micro batteries display superior capacity, satisfactory rate performance and long-term durability.. show the positive effects of a mesoporous structure on achieving optimized electrochemical performance by developing mesoporous nitrogen-doped carbon/graphene nanosheets using aniline as the precursor, graphene oxide as a guiding agent and silica spheres as a mesoporous template, which was highlighted as cover.summarized the recent advances of graphene and graphene-based materials for in-plane micro capacitors to enhance energy density and promote practical applications. The 2D surface of graphene with the in-plane architecture helps to improve the ion accessibility of electrodes. The prospects of graphene for in-plane micro capacitors are proposed, hoping to inspire future endeavors in this field and promote their practical application. To overcome the low mass loading of active materials and less-than-perfect energy density of Li-ion micro batteries,. designed 3D printed ZnSe/N-doped carbon electrode by the extrusion-based 3D printing and post-treatment. This structure improves electrical conductivity, alleviates the volume expansion and provides additional active sites for electrochemical reactions. Therefore, 3D printing with its capability to build geometrically complex 3D architectures, enables the manufacturing of microbatteries with dimensional tunability, excellent shape conformability, and simultaneously high energy and power density..reviewed the recent progress of the four major 3D printing techniques, elaborating on the use of conductive carbon materials in studies that address challenging issues of 3D printed micro batteries, and summarized their applications in a series of energy storage devices and integration with wearable electronics.

In summary, carbon-based materials have played a vital role in achieving state-of-the-art flexible and/or micro energy storage/conversion devices, including various types of electrochemical capacitors and rechargeable batteries and solar cells. And considerable effort has been dedicated to developing carbon materials including porous carbons, carbon cloth, carbon nanotubes, graphene, and carbon dots etc. Nonetheless, the development of carbon materials for flexible and/or micro energy devices is still in its infancy, especially when considering the practical applications. From a materials perspective, rational selection,and tailored synthesis of target-orientated carbon materials according to their respective function/role in flexible or micro energy devices are still ongoing endeavors to keep pace with the evolving criteria. Proper electrode and device architecture must employ newly-emerging technologies for better fabrication and integration, enhanced ion storage/transport with superior power and energy density and flexibility compatible with wearable devices. In this regard, the proper balance of high intrinsic conductivity, porosity, flexibility, and interfacial compatibility of carbons with other active materials, electrodes, and separators is highly recommended. Meanwhile, precise simulations could help analyze the effect of microsizing and deformation on the overall performance. In addition,building self-powered integrated flexible and/or micro systems by combining energy-harvesting devices, like solar cells and nanogenerators will be a thriving area of research catalyzing the rapid advances of intelligent electronic devices. Last but not the least, large-scale, time-efficient, high-throughput and lowcost fabrication technology should be promoted. The importance of solving safety issues like electrolyte leakage under the deformation of flexible units cannot be understated. The rapid advances in the design and application of carbon materials for flexible and/or micro energy storage/conversion devices has excited scientists and engineers from different fields, which has evolved this field into a hotbed of interdisciplinary endeavors. We hope this will trigger a cascade of new, refreshing ideas in both fundamental understanding and practical applications of carbonbased flexible and micro energy devices.