Nickel oxide-encapsulated hollow carbon nitride spheres with multiporosity show an ∼250% enhancement in capacitance, in addition to their robust cycle life.

Nanocrystals are promising structures, but they are too large for achieving maximum energy storage performance. We show that rescaling 3-nm particles through lithiation followed by delithiation leads to high-performance energy storage by realizing high capacitance close to the theoretical capacitance available via ion-to-atom redox reactions. Reactive force-field (ReaxFF) molecular dynamics simulations support the conclusion that Li atoms react with nickel oxide nanocrystals (NiO-n) to form lithiated core-shell structures (Ni:Li 2 O), whereas subsequent delithiation causes Ni:Li 2 O to form atomic clusters of NiO-a. This is consistent with in situ X-ray photoelectron and optical spectroscopy results showing that Ni 2+ of the nanocrystal changes during lithiation-delithiation through Ni 0 and back to Ni 2+ . These processes are also demonstrated to provide a generic route to rescale another metal oxide. Furthermore, assembling NiO-a into the positive electrode of an asymmetric device enables extraction of full capacitance for a counter negative electrode, giving high energy density in addition to robust capacitance retention over 100,000 cycles.rescaled atomic clusters | metal oxide nanocrystals | energy storage | molecular dynamic simulation | in situ electrochemical spectroscopy T he most critical challenge in energy storage is maximizing capacitance along with high power density and long cycle life. High-power capacitors (1-4) are candidates to meet this challenge, and can be classified into two categories: (i) energy storage systems where charge is stored in electrochemical double layers (EDLs) (5, 6) and (ii) pseudocapacitors that store charge by redox reactions (7-14). Unfortunately, EDLs have low capacitance, whereas metal oxide pseudocapacitors lead to short cycle life. Furthermore, typical capacitors have low energy density (5, 10). In principle, the capacitances of metal oxide crystals can be fully obtained via ion-by-atom surface redox reactions. A capacitor that enables high capacitance with high energy density and long cycle life thus would represent a major breakthrough in energy storage.We synthesized metal oxide nanocrystals at a size of several nanometers on graphene, but found that rapid charging-discharging achieves only about 15% of their full capacitance. We hypothesized that reducing their sizes to the atomic clusters of subnanometer scales less than 1 nm, combined with conducting flexible graphene, would allow full redox reactions over entire constituents. Here we report that lithiation of 3-nm nickel oxide nanocrystals on graphene (NiO-n/gr) causes them to rescale down to subnanometer-scale Ni:Li 2 O-a/gr core-shell clusters and that subsequent delithiation of Ni:Li 2 O core-shell clusters leads to NiO (NiO-a/gr). We established the sequence as follows:NiO-n=gr → Ni : Li 2 O-a=gr → NiO-a=gr.We then verified this using a combination of experimental characterization with complementary reactive molecular dynamics.Moreover, we show that loading a positive electrode with NiO-a particles into an...

energy densities (100-160 Wh kg −1), they suffer from both slow charging rates attributed to their low power densities (<1000 W kg −1) and also short cycle stability. [3,4] As alternative energy storage systems (ESS), electrochemical capacitors (ECs) allow higher power densities and better cycle stability than LIBs, but typical ECs allow extremely small energy densities due to the surface-limited charge-discharge redox reactions. [5] Hence, the sole usage of a LIB or an EC does not give simultaneously high energy and power densities along with long cycle stability because of their complementary ion storage mechanisms. As a solution to this challenge, hybrid energy storages (HESs), where charges are asymmetrically stored by battery-type reactions in the anode and pseudocapacitive reactions in the cathode, are of great attraction as their charge and discharge processes can be controlled using different potential windows to increase energy density. [6-9] However, despite this promise, an anode structure is having both rich active sites for high capacity and also fast cation transport channels and electron conduction pathways, [10-13] and its compatible cathode structure is allowing both rapid anion movement and facile electron conduction and also rich ion adsorption and pseudocapacitive reactions are vital to realizing high energy density for prolonged operation in a single charge and high power density for fast chargeable capability in the HES full cell device. [14-16] In this work, we synthesize the mesoporous molybdenum dioxide structures derived from metal-organic frameworks (MOFs) coated with reduced graphene oxide (rGO) shells as high-capacity and high-rate anode structures (MoO 2 @rGO) and the in situ polymerized crosslinked polyaniline (PANI) chain-integrated rGO structures as high-capacity and high-rate cathode structures (PANI@rGO). MoO 2 has a high theoretical capacity (838 mAh g −1), efficient charge stability with the low electrical resistivity (≈8.8 × 10 −5 Ω cm −1 at 300 K), and excellent chemical stability. [17] However, to utilize MoO 2 as an anode material unit for high-capacity, its poor diffusivity and conductivity Hybrid lithium-ion energy storage devices are promising for future applications, but their anodes and cathodes still have structural limitations, for example, accommodating rich cationic/anionic reactions, rapid charge movement, and long cycle life. Herein, high-capacity/high-rate anode and cathode structures are developed to overcome these challenges. Molybdenum oxide (MoO 2)-implanted carbon frameworks making conductive carbon bonds with reduced graphene oxide (rGO) shells are developed as anode structures by forming mesoporous channels for fast lithium-ion transport, carbon-rGO pathways for facile electron conduction, and ultrafine MoO 2 units for high capacity. The operando X-ray diffraction and kinetics analyses reveal that lithium-ion insertion and extraction occur via capacitive and diffusioncontrolled reactions. Also, polyaniline (PANI) chains are elongated on rGO sheets throug...

Realization of safe electrochemical energy storages with high energy density and long cycle life along with the high power density enabling fast charging is a major challenge. Here, a strategy to realize high‐performance aqueous energy storages using porous Mn3O4 (p‐MG) positive and porous Fe2O3 (p‐FG) negative electrodes, where granular nanoclusters composing nanoparticles are produced on graphene through lithiation‐induced conversion and the shortened ion diffusion lengths in p‐MG and p‐FG give fast charging rate and excellent cycle stability is reported. Furthermore, it is found from cyclic voltammetry curves and specific capacitances that porous metal oxide structures play mainly as redox reaction sites, while graphene structures provide electrical conductivity to active sites. Indeed, the full‐cell configuration of p‐MG and p‐FG in a hybrid capacitor exhibits a distinguished high energy density exceeding those of aqueous batteries, in addition to excellent capacity retention over 30 000 redox cycles and the energy density 2.5‐fold higher than that of its counterpart with pristine Mn3O4 and Fe2O3 nanocrystals. Additionally, this capacitor shows the high power density allowing ultrafast charging in that the full cells in series can be charged within several seconds by the rapid USB charger, thus outperforming those of typical aqueous batteries by about 100‐fold.

friendly, and safe electrochemical energy storage is growing. Conventional LIBs possess relatively high-energy density but suffer from slow charging rates owing to their low power density and short cycle stability. There are also safety hazards associated with the use of these environmentally toxic and flammable material. Moreover, these LIBs require expensive organic electrolytes to maintain the high voltages required for high-energy density. [3,4] Hence, low-cost aqueous ECs, which can operate in environmentally friendly and safe aqueous electrolytes, have been developed as candidates to overcome the aforementioned challenges of organic electrolyte-based electrochemical energy storage. Nevertheless, their low capacity and working voltage limit high-energy density for advanced use. [5,6] Aqueous hybrid capacitors (AHCs) present a promising alternative as electrochemical energy storage systems and can be designed as asymmetric full-cells using the different potential windows of anode and cathode reactions for high-energy density. [7-9] Earth-abundant metal oxides are promising materials for atom-by-ion reactions, but their low conductivity limits full capacitance, resulting in a slow charging rate and poor cycle stability. [10-12] This suggests that conductive metal sulfides are suitable electrode materials for a fast charging rate and long-life cycle stability. [6,13-15] Moreover, the synthesis of cathode materials at nanometer-scale sizes is required to enlarge active surface areas for high capacity and to shorten diffusion lengths for fast ion transfer; however, the random dense packing and agglomeration in electrodes lead to low tap density and capacity fading during repeated charge-discharge cycles. [16,17] Therefore, the development of a new cathode material to promote high tap density and long-life stability remains an enormous challenge. Moreover, very few materials capable of high-energy density have been synthesized as anode materials compatible with cathode materials. [18-20] Reduced graphene oxide aerogel (rGOA), which is a three-dimensional (3D) self-assembled material with rGO sheets, is a novel anode material that possesses ultra-light density, hierarchical pores for fast ion transport, and carbon networks for facile electron conduction. [18,19] However, rGOA anode gives a low capacity due to its limited active Aqueous hybrid capacitors (AHCs) are very promising electrochemical energy devices due to their being safe, cheap, and environmentally friendly, but their low energy and power densities are yet to be overcome for prolonged operation in a single fast charging device. Herein, a strategy to realize high-energy density and ultrafast rechargeable AHCs is reported. A thorn-covered core-shell conductive multivalence metal sulfide is synthesized as a cathode material that achieves high capacity via multivalence Ni and Co states and contains multiple mesoporous channels for fast ion transfer and ultrafine nanoparticles for efficient contact with electrolyte ions. Moreover, the multivalence metal stat...