Two-dimensional birnessite has attracted attention for electrochemical energy storage because of the presence of redox active Mn/Mn ions and spacious interlayer channels available for ions diffusion. However, current strategies are largely limited to enhancing the electrical conductivity of birnessite. One key limitation affecting the electrochemical properties of birnessite is the poor utilization of the MnO unit. Here, we assemble β-MnO/birnessite core-shell structure that exploits the exposed crystal face of β-MnO as the core and ultrathin birnessite sheets that have the structure advantage to enhance the utilization efficiency of the Mn from the bulk. Our birnessite that has sheets parallel to each other is found to have unusual crystal structure with interlayer spacing, Mn(III)/Mn(IV) ratio and the content of the balancing cations differing from that of the common birnessite. The substrate directed growth mechanism is carefully investigated. The as-prepared core-shell nanostructures enhance the exposed surface area of birnessite and achieve high electrochemical performances (for example, 657 F g in 1 M NaSO electrolyte based on the weight of parallel birnessite) and excellent rate capability over a potential window of up to 1.2 V. This strategy opens avenues for fundamental studies of birnessite and its properties and suggests the possibility of its use in energy storage and other applications. The potential window of an asymmetric supercapacitor that was assembled with this material can be enlarged to 2.2 V (in aqueous electrolyte) with a good cycling ability.

Because of the great breakthroughs of self‐healing materials in the past decade, endowing devices with self‐healing ability has emerged as a particularly promising route to effectively enhance the device durability and functionality. This article summarizes recent advances in self‐healing materials developed for energy harvesting and storage devices (e.g., nanogenerators, solar cells, supercapacitors, and lithium‐ion batteries) over the past decade. This review first introduces the main self‐healing mechanisms among different materials including insulators, electrical conductors, semiconductors, and ionic conductors. Then, the basic concepts, fabrication techniques, and healing performances of the newly developed self‐healing energy harvesting (nanogenerators and solar cells) and storage (supercapacitors and lithium‐ion batteries) devices are described in detail. Finally, the existing challenges and promising solutions of self‐healing materials and devices are discussed.

In the past few years, intensive attention has been focused on birnessite based electrodes for supercapacitors.

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The excessive concern over the energy density of supercapacitors is changing their applied direction while the power density is always overlooked. Supercapacitors should be considered as a high-power energy device rather than a neither fish nor fowl energy device. Herein, an ultrafine α-MnO 2 needle was formed on β-MnO 2 networks, not distributed randomly, but standing on the surface of β-MnO 2 vertically forming an array structure with low-charge-carrier scattering. These novel structures possess a rational arrangement of the needles resulting in high capacitance (278.2 F g −1 for α-MnO 2 /β-MnO 2 networks) and excellent rate capability (41.0% remaining with the specific current increased from 0.25 to 64.0 A g −1 ). Asymmetrical supercapacitors fabricated by reduced graphene oxide (RGO) as the anode and as-prepared structures as the cathode deliver excellent electrochemical performance. Specifically, the devices give a favorable specific energy (29.8 W h kg −1 ) considering the weight of α-MnO 2 /β-MnO 2 and reduced graphene oxide (RGO) as well as ultrahigh specific power (64.0 kW kg −1 ) and excellent cyclic stability (>95% of initial capacitance remained after 10 000 cycles). This work opens new avenues for promoting high-power asymmetrical supercapacitors.