Low-crystalline Ni/Co-oxyhydroxides nanoarrays on carbon cloth with high mass loading and hierarchical structure as cathode for supercapacitors

One-Step Hydrothermal Synthesis of Glucose-Induced Low Crystallinity NiCo-Based Layered Double Hydroxides for High-Performance Asymmetric Supercapacitors

In order to improve the electrochemical performance of NiCo-based layered double hydroxide (NiCoLDH), the synthesis of low-crystallinity NiCoLDH was induced by the adsorption of glucose and NiCoLDH. The results showed that glucose could not only effectively regulate the pore structure and morphology of NiCoLDH, but also had a regular effect on crystallinity. Pure phase NiCoLDH had higher crystallinity. When the mass of glucose is 0.05 g, the prepared NiCoLDH-0.05 is a short-range ordered structure embedded in the amorphous matrix. The crystallinity of the product decreases further with the further increase of glucose mass. Since the ordered structures have higher electrical conductivity, and amorphous structures have more defects and active sites, the structure of NiCoLDH-0.05 is conducive to achieving the best electrochemical performance. Electrochemical test results show that NiCoLDH-0.05 has a high specific capacitance, about 12 times that of the pure phase NiCoLDH, the mass of glucose is higher than or below 0.05 g, the specific capacitance will be further reduced. NiCoLDH-0.05 and activated carbon assembled into an asymmetric supercapacitor have a power density of 400 W kg-1 at an energy density of 32.7 Wh kg-1. This study provides a new idea for obtaining excellent electrochemical properties by adjusting LDH crystallinity.

In Situ Fabrication of Hierarchical CuO@CoNi-LDH Composite Structures for High-Performance Supercapacitors

Layered double hydroxide (LDH) materials, despite their high theoretical capacity, exhibit significant performance degradation with increasing load due to their low conductivity. Simultaneously achieving both high capacity and high rate performance is challenging. Herein, we fabricated vertically aligned CuO nanowires in situ on the copper foam (CF) substrate by alkali-etching combined with the annealing process. Using this as a skeleton, electrochemical deposition technology was used to grow the amorphous α-phase CoNi-LDH nanosheets on its surface. Thanks to the high specific surface area of the CuO skeleton, ultrahigh loading (̃16.36 mg cm-2) was obtained in the fabricated CF/CuO@CoNi-LDH electrode with the cactus-like hierarchical structure, which enhanced the charge transfer and ion diffusion dynamics. The CF/CuO@CoNi-LDH electrode achieved a good combination of high areal capacitance (33.5 F cm-2) and high rate performance (61% capacitance retention as the current density increases 50 times). The assembled asymmetric supercapacitor device demonstrated a maximum potential window of 0-1.6 V and an energy density of 1.7 mWh cm-2 at a power density of 4 mW cm-2. This work provides a feasible strategy for the design and fabrication of high-mass-loading LDH composites for electrochemical energy storage applications.

Honeycomb porous heterostructures of NiMo layered double hydroxide nanosheets anchored on CoNi metal-organic framework nano-blocks as electrodes for asymmetric supercapacitors

Supercapacitors (SCs) have the advantages of high power density, long cycle life, and fast charging/discharging rates, but relatively low energy density limits their practical application prospects. The key to improving the energy density of supercapacitors is to develop electrode materials with excellent performance. Metal-organic frameworks (MOFs) used for electrochemical energy storage have emerged as a research hotspot due to their adjustable microstructure, porosity, and high specific surface area. To address the demands of high-performance supercapacitors, composite nanomaterials can be prepared by rationally designing MOFs. Herein, CoNi-MOF nano-blocks are grown on the carbon cloth, and ultrathin NiMo layered double hydroxides (NiMo-LDH) nanosheets are further anchored on its surfaces to form a honeycomb porous heterostructure (NiMo-LDH@CoNi-MOF). The porous heterostructures increase the electrochemically active specific surface area and shorten the charge transfer distance, possessing ultra-high capacitance of 15.6 F/cm2 at 1 mA/cm2. Furthermore, utilizing annealed activated carbon cloth (AAC) as the negative electrode, the assembled NiMo-LDH@CoNi-MOF-2//AAC asymmetric supercapacitor possesses an energy density of 1.10 mWh/cm2 at a power density of 4 mW/cm2, and a capacitance retention of 97.8 % after 10,000 cycles. This material exhibits distinctive nanostructures and favorable electrochemical characteristics, providing a unique idea for preparing supercapacitors with high energy density and power density.

Transforming the Electrochemical Behaviors of Cobalt Oxide from "Supercapacitator" to "Battery" by Atomic-Level Structure Engineering for Inspiring the Advance of Co-Based Batteries

Cobalt-based electrodes receive emerging attention for their high theoretical capacity and rich valence variation ability, but state-of-the-art cobalt-based electrodes present performance far below the theoretical value. Herein, the in-depth reaction mechanisms in the alkaline electrolyte are challenged and proven to be prone to the surface-redox pseudocapacitor behavior due to the low adsorption energy to OH. Using the atomic-level structure engineering strategy after substitution metal searching, the adsorption energy is effectively enhanced, and the peak of CoOOH can be observed from in situ characterization for the first time, leading to the successful transition of charge storage behavior from "supercapacitor" to "battery". When used in a Zn-Co battery as a proof of concept, it shows comprehensive electrochemical performance with a flat discharge voltage plateau of ≈1.7 V, an optimal energy density of 506 Wh kg^-1 , and a capacity retention ratio of 85.1% after 2000 cycles, shining among the reported batteries. As a practical demonstration, this battery also shows excellent self-discharge performance with the capacity retention of 90% after a 10 h delay. This work subtly tunes the intrinsic electrochemical properties of the cobalt-based material through atomic-level structure engineering, opening a new opportunity for the advance of energy storage systems.

3D hollow NiCo LDH nanocages anchored on 3D CoO sea urchin-like microspheres: A novel 3D/3D structure for hybrid supercapacitor electrodes

The research on the structure of advanced electrode materials is significant in the field of supercapacitors. Herein, for the first time, we propose a novel 3D/3D composite structure by a multi-step process, in which 3D hollow NiCo LDH nanocages are immobilized on 3D sea urchin-like CoO microspheres. Results show that the 3D CoO acts as an efficient and stable channel for ion diffusion, while the hollow NiCo LDH provides abundant redox-active sites. The calculated results based on density function theory (DFT) show that the CoO@NiCo LDH heterostructure has an enhanced density of states (DOS) near the Fermi level and strong adsorption capacity for OH^-, indicating its excellent electrical conductivity and electrochemical reaction kinetics. As a result, the CoO@NiCo LDH electrode has an areal specific capacity of 4.71C cm^-2 at a current density of 3 mA cm^-2 (440.19C g^-1 at 0.28 A g^-1) and can still maintain 88.76 % of the initial capacitance after 5000 cycles. In addition, the assembled hybrid supercapacitor has an energy density of 5.59 mWh cm^-3 at 39.54 mW cm^-3.

LiNi0.6Co0.2Mn0.2O2 Cathodes Coated with Dual-Conductive Polymers for High-Rate and Long-Life Solid-State Lithium Batteries

High-energy density and safe solid lithium batteries call for cathodes with high capacity and good kinetic properties. In this work, LiNi_0.6Co_0.2Mn_0.2O₂ (NCM622) cathodes are coated with the ionic-electronic dual-conductive polymers composed of poly(ethylene glycol) (PEG)-doped polyaniline (PANI). Scanning electron microscopy, transmission electron microscopy, Fourier transform infrared, and thermogravimetric analysis reveal that the dual-conductive polymers are homogeneously coated on the surfaces of NCM622 cathodes with a thickness of approximately 10 nm. The solid-state lithium batteries consisting of the NCM622 cathodes coated with PANI-PEG show a specific capacity of 158 mA h g^-1 and a retention rate of 88% after 100 cycles at the rate of 0.1 C and room temperature, which are superior to the discharge capacity of 153 mA h g^-1 and capacity retention of 59% after 100 cycles for the batteries with the pristine NCM622 cathodes. Moreover, the cells with the coated cathodes display a better rate performance of 84 mA h g^-1 at 1 C than those with the uncoated ones which show a rate performance of 11 mA h g^-1 at 1 C.

Synthesis and Study on Ni-Co Phosphite/Activated Carbon Fabric Composited Materials with Controllable Nano-Structure for Hybrid Super-Capacitor Applications

The advantage of low resistivity and inactive binders makes binder-free electrode an excellent candidate for high-performance energy devices. A simple hydrothermal method was used to fabricate M11(HPO3)8(OH)6 (M: Ni and Co) (MHP) arrays combined with activated carbon fabric (ACF) without binder. The structures of MHP can be easily tuned from bouquets to nano-sheets by the concentration of NaH2PO2. The MHP/ACF composite materials with different structures showed the typical battery-type characteristic of anodic electrodes. In a three-electrode cell configuration, the MHP nano-sheet arrays/ACF composite has a higher capacity, of 1254 F/g, at a scan rate of 10 mA/cm2 and shows better cycling stability: 84.3% remaining specific capacity after 1000 cycles of charge-discharge measurement. The composite is highly flexible, with almost the same electrochemical performance under stretching mode. The MHP/ACF composite@ACF hybrid supercapacitor can deliver the highest energy density, of 34.1 Wh·kg-1, and a power density of 722 W·kg-1 at 1 A·g-1. As indicated by the results, MHP/ACF composite materials are excellent binder-free electrodes, candidates for flexible high-performance hybrid super-capacitor devices.

Recent Developments of Transition Metal Compounds-Carbon Hybrid Electrodes for High Energy/Power Supercapacitors

Due to their rapid power delivery, fast charging, and long cycle life, supercapacitors have become an important energy storage technology recently. However, to meet the continuously increasing demands in the fields of portable electronics, transportation, and future robotic technologies, supercapacitors with higher energy densities without sacrificing high power densities and cycle stabilities are still challenged. Transition metal compounds (TMCs) possessing high theoretical capacitance are always used as electrode materials to improve the energy densities of supercapacitors. However, the power densities and cycle lives of such TMCs-based electrodes are still inferior due to their low intrinsic conductivity and large volume expansion during the charge/discharge process, which greatly impede their large-scale applications. Most recently, the ideal integrating of TMCs and conductive carbon skeletons is considered as an effective solution to solve the above challenges. Herein, we summarize the recent developments of TMCs/carbon hybrid electrodes which exhibit both high energy/power densities from the aspects of structural design strategies, including conductive carbon skeleton, interface engineering, and electronic structure. Furthermore, the remaining challenges and future perspectives are also highlighted so as to provide strategies for the high energy/power TMCs/carbon-based supercapacitors.