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the demands for energy storage and conversion systems of high energy and power density increase rapidly. To meet the everincreasing demands, transition metal compounds have been widely studied as catalysts for chemical and energy transformation processes (e.g., oxygen reduction and evolution reaction) [1][2][3][4] and as electrode materials for rechargeable batteries [5][6][7] and supercapacitors. [8][9][10][11][12][13] For example, nickel hydroxides (Ni(OH) 2 ) have been successfully used in rechargeable alkaline batteries (due to their low-cost and high capacity) [5,14] and in hybrid supercapacitors (due to high rate capability). [15] Hybrid supercapacitors, composed of a capacitive electrode and a battery-type Faradaic electrode, have demonstrated significantly higher energy density than conventional carbon-based electrical double-layer capacitors (EDLCs), due largely to the higher capacity of the battery-type electrode and the broader voltage window of the electrode pair. [16][17][18][19][20] Moreover, hybrid supercapacitors can achieve much higher power density than rechargeable batteries. Typically, one electrode of a hybrid supercapacitor is a carbon-based material whereas the other electrode (i.e., battery-type electrode) is a lithium electroactive material (such as Sn [16] and Li 4 Ti 5 O 12 [21] ) or an anionic redox active transition metal-based oxide/hydroxide. [15,22] As a promising battery-type electrode for hybrid supercapacitor in alkaline electrolyte, Ni(OH) 2 usually exhibits a pair of distinctly separated Faradaic redox peaks due to its phase transition. [23] This material was widely reported as a "pseudocapacitive" material for supercapacitors in the past decade, [24][25][26][27][28][29][30] but has recently been regarded as a battery-type material because of its batterytype behavior in alkaline media. [31][32][33] However, Ni(OH) 2 electrodes usually suffer from an irreversible phase transition, [34,35] large volume variation, [14] and low electronic conductivity, [36] resulting in poor durability and limited rate capability.Numerous efforts have been devoted to addressing the above issues. In order to enhance the electronic conductivity, an electronically conductive second phase has been introduced [15,24,37] or Ni(OH) 2 has been grown on a conductive substrate. [28,38,39] As a highly conductive two-demonstrational material with high surface area, graphene is an ideal substrate to improve the electronic conductivity of Ni(OH) 2 . [15,24,40] To further improve Compact, light, and powerful energy storage devices are urgently needed for many emerging applications; however, the development of advanced power sources relies heavily on advances in materials innovation. Here, the findings in rational design, one-pot synthesis, and characterization of a series of Ni hydroxide-based electrode materials in alkaline media for fast energy storage are reported. Under the guidance of density functional theory calculations and experimental investigations, a composite electrode composed of Co-/Mn-substituted...

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Metallic lithium has long been deemed as the ultimate anode material for future high‐energy‐density Li batteries. However, the commercialization of Li metal anodes remains hindered by some major hurdles including their huge volume fluctuation during cycling, unstable solid electrolyte interface (SEI), and dendritic deposition. Herein, the concept of nano‐encapsulating electrode materials is attempted to tackle these problems. Nitrogen‐doped hollow porous carbon spheres (N‐HPCSs), prepared via a facile and low‐cost method, serve as the nanocapsules. Each N‐HPCS has a lithophilic carbon shell with a thin N‐rich denser layer on its inner surface, which enables preferential nucleation of Li inside the hollow sphere. It is demonstrated by in situ electron microscopy that these N‐HPCS hosts allow Li to be encapsulated in a highly reversible and repeatable manner. Ultralong Li filling/stripping cycling inside single N‐HPCSs is achieved, up to 50 cycles for the first time. Li ion transport across multiple connected N‐HPCSs, leading to long‐range Li deposition inside their cavities, is visualized. In comparison, other types of carbon spheres with modified shell structures fail in encapsulating Li and dendrite suppression. The necessity of the specific shell design is therefore confirmed for stable Li encapsulation, which is essential for the N‐HPCS‐based anodes to achieve superior cycling performance.

Li-rich layered oxides have been in focus because of their high specific capacity. However, they usually suffer from poor kinetics, severe voltage decay, and capacity fading. Herein, a long-neglected Li-deficient method is demonstrated to address these problems by simply reducing the lithium content. Appropriate lithium vacancies can improve dynamics features and induce in situ surface spinel coating and nickel doping in the bulk. Therefore, the elaborately designed Li 1.098 Mn 0.533 Ni 0.113 Co 0.138 O 2 cathode possesses improved initial Coulombic efficiency, excellent rate capability, largely suppressed voltage decay, and outstanding long-term cycling stability. Specifically, it shows a superior capacity retention of 93.1% after 500 cycles at 1 C (250 mA g −1 ) with respect to the initial discharge capacity (193.9 mA h g −1 ), and the average voltage still exceeds 3.1 V. In addition, the discharge capacity at 10 C can be as high as 132.9 mA h g −1 . More importantly, a Li-deficient cathode can also serve as a prototype for further performance enhancement, as there are plenty of vacancies.