Carbon nanotube@manganese oxide nanosheet core-shell structure encapsulated within reduced graphene oxide film for flexible all-solid-state asymmetric supercapacitors

To achieve flexible energy storage devices with high performance, a new class of flexible electrodes with exquisite architectures that provide well-defined pathways for efficient ionic and electronic transport is highly needed. A flexible 3D electrode is fabricated based on assembling 1D carbon nanotube@manganese oxide (MnO2) nanosheet core-shell structures (CM) with 2D reduced graphene oxide (rGO) nanosheets. MnO2 nanosheets are grafted vertically along the carbon nanotube backbone through a secondary (seeded) growth. Flexible hybrid films (GCM) composed of CM and rGO are prepared by vacuum filtration of the mixed dispersions of both components assisted by electrostatic interactions. By adopting this ternary hybrid architecture, the GCM electrode exhibits exceptional charge storage capability in 1 M Na2SO4 aqueous electrolyte with high specific capacitance (298 F g−1 at 0.5 A g−1), good rate capability, and high cycling stability (90.3% retention over 5000 cycles). A highly flexible all-solid-state asymmetrical supercapacitor is assembled with GCM as the positive electrode and holey graphene film spaced with carbon sphere (H-GCS) as the negative electrode. GCM//H-GCS asymmetric supercapacitor delivers a maximum energy density of 24.6 Wh kg−1 and a power density of 9005.3 W kg−1 with excellent cycle life of 75.9% retention after 10 000 cycles

Electrostatic-induced assembly of graphene-encapsulated carbon@nickel-aluminum layered double hydroxide core-shell spheres hybrid structure for high-energy and high-power-density asymmetric supercapacitor

Achieving high energy density while retaining high power density is difficult in electrical double-layer capacitors and in pseudocapacitors considering the origin of different charge storage mechanisms. Rational structural design became an appealing strategy in circumventing these trade-offs between energy and power densities. A hybrid structure consists of chemically converted graphene-encapsulated carbon@nickel-aluminum layered double hydroxide core–shell spheres as spacers among graphene layers (G-CLS) used as an advanced electrode to achieve high energy density while retaining high power density for high-performance supercapacitors. The merits of the proposed architecture are as follows: (1) CLS act as spacers to avoid the close restacking of graphene; (2) highly conductive carbon sphere and graphene preserve the mechanical integrity and improve the electrical conductivity of LDHs hybrid. Thus, the proposed hybrid structure can simultaneously achieve high electrical double-layer capacitance and pseudocapacitance resulting in the overall highly active electrode. The G-CLS electrode exhibited high specific capacitance (1710.5 F g−1 at 1 A g−1) under three-electrode tests. An ASC fabricated using the G-CLS as positive electrode and reduced graphite oxide as negative electrode demonstrated remarkable electrochemical performance. The ASC device operated at 1.4 V, and delivered a high energy density of 35.5 Wh kg−1 at a 670.7 W kg−1 power density at 1 A g−1 with an excellent rate capability, as well as a robust long-term cycling stability of up to 10 000 cycles

Ultrathin petal-like NiAl layered double oxide/sulfide composites as an advanced electrode for high-performance asymmetric supercapacitors

Layered double hydroxide (LDH) is an important layer-structured material for supercapacitors because of its versatile compositions, high theoretical capacitance, environmental friendliness, and low cost. However, the high resistivity of this material results in capacity fading, limiting its application in energy storage. Herein, we develop a facile approach to synthesize ultrathin petal-like NiAl layered double oxide/sulfide (LDO/LDS) composites with high electrochemical activity using hydrothermal reaction followed by sulfidation process. Scanning electron micrograph shows that the petal-like NiAl LDO/LDS composites are as thin as ~10 nm with a mean lateral dimension of ~1 µm. The NiAl LDO/LDS electrode delivers remarkably high specific capacitance of 2250.5 F g−1 at 1 A g−1 compared with that of NiAl LDH (1740.5 F g−1 at 1 A g−1) and possesses good cycling ability of 88.9% capacitance retention over 5000 cycles at 5 A g−1. Asymmetric supercapacitor (ASC) is fabricated using NiAl LDO/LDS and graphene as positive and negative electrodes, respectively. NiAl LDO/LDS//G ASC exhibits specific capacitance of 153.3 F g−1 at 1 A g−1, high energy density of 47.9 Wh kg−1 at a power density of 750 W kg−1, and reliable cycling stability of 95.68% capacitance retention after 5000 cycles. Results highlight that NiAl LDO/LDS composites are promising materials for energy storage devices with long cycling stability

Supercapacitor Performance of Nickel-Cobalt Sulfide Nanotubes Decorated Using Ni Co-Layered Double Hydroxide Nanosheets Grown in Situ on Ni Foam

In this study, to fabricate a non-binder electrode, we grew nickel-cobalt sulfide (NCS) nanotubes (NTs) on a Ni foam substrate using a hydrothermal method through a two-step approach, namely in situ growth and an anion-exchange reaction. This was followed by the electrodeposition of double-layered nickel-cobalt hydroxide (NCOH) over a nanotube-coated substrate to fabricate NCOH core-shell nanotubes. The final product is called NCS@NCOH herein. Structural and morphological analyses of the synthesized electrode materials were conducted via SEM and XRD. Different electrodeposition times were selected, including 10, 20, 40, and 80 s. The results indicate that the NCSNTs electrodeposited with NCOH nanosheets for 40 s have the highest specific capacitance (SC), cycling stability (2105 Fg-1 at a current density of 2 Ag-1), and capacitance retention (65.1% after 3,000 cycles), in comparison with those electrodeposited for 10, 20, and 80 s. Furthermore, for practical applications, a device with negative and positive electrodes made of active carbon and NCS@NCOH was fabricated, achieving a high-energy density of 23.73 Whkg-1 at a power density of 400 Wkg-1

Recent advances in the interface design of solid-state electrolytes for solid-state energy storage devices

High-ionic-conductivity solid-state electrolytes (SSEs) have been extensively explored for electrochemical energy storage technologies because these materials can enhance the safety of solid-state energy storage devices (SSESDs) and increase the energy density of these devices. In this review, an overview of SSEs based on their classification, including inorganic ceramics, organic solid polymers, and organic/inorganic hybrid materials, is outlined. Related challenges, such as low ionic conductivity, high interfacial resistance between electrodes and SSEs, poor wettability, and low thermal stability, are discussed. In particular, recent advances in properties of SSEs and interface design of high-performance SSESDs are highlighted. Several interface designs, including hybrid, interlayer, solid-liquid, quasi-solid-state gel, and in situ solidification interface, between electrodes and SSEs for alleviating interfacial resistance, stability, and compatibility in SSESDs are comprehensively reviewed to provide insights into the future design directions of SSEs and SSESDs. The rational designs of various SSESDs for flexible and wearable devices, electronic devices, electric vehicles, and smart grid systems are proposed in accordance with different practical application requirements

Hierarchical ultrathin NiAl layered double hydroxide nanosheet arrays on carbon nanotube paper as advanced hybrid electrode for high performance hybrid capacitors

To effectively improve the power density and rate capability of layered double hydroxide (LDH) based supercapacitors, a hybrid supercapacitor (HSC) comprising of hierarchical ultrathin NiAl-LDH nanosheet arrays on carbon nanotube paper (CNP-LDH) is developed with porous graphene nanosheets as the negative electrode for the first time. SEM image shows that hierarchical NiAl LDH nanosheet arrays are assembled by numerous ultrathin nanosheets with thickness of a few to tens of nanometers. Remarkably, with an operating voltage of 1.6 V, the HSC possesses a high energy density of 50.0 Wh kg-1 at an average power density of 467 W kg-1. Even at a fast discharging time of 3.9 s, a high energy density (23.3 Wh kg-1) could also be retained at a power density of 21.5 kW kg-1. Moreover, the HSC exhibits cycling stability with a retention rate of 78% after 5000-cycle charge-discharge test at 5 A g-1. The results inspire us to propose our high-performance CNP-LDH as a promising electrode for energy storage applications

Hierarchical chestnut-like MnCo2O4 nanoneedles grown on nickel foam as binder-free electrode for high energy density asymmetric supercapacitors

Hierarchical chestnut-like manganese cobalt oxide (MnCo2O4) nanoneedles (NNs) are successfully grown on nickel foam using a facile and cost-effective hydrothermal method. High resolution TEM image further verifies that the chestnut-like MnCo2O4 structure is assembled by numerous 1D MnCo2O4 nanoneedles, which are formed by numerous interconnected MnCo2O4 nanoparticles with grain diameter of ∼10 nm. The MnCo2O4 electrode exhibits high specific capacitance of 1535 F g−1 at 1 A g−1 and good rate capability (950 F g−1 at 10 A g−1) in a 6 M KOH electrolyte. An asymmetric supercapacitor is fabricated using MnCo2O4 NNs on Ni foam (MnCo2O4 NNs/NF) as the positive electrode and graphene/NF as the negative electrode. The device shows an operation voltage of 1.5 V and delivers a high energy density of ∼60.4 Wh kg−1 at a power density of ∼375 W kg−1. Moreover, the device exhibits an excellent cycling stability of 94.3% capacitance retention after 12000 cycles at 30 A g−1. This work demonstrates that hierarchical chestnut-like MnCo2O4 NNs could be a promising electrode for the high performance energy storage devices

Recent advances in hybrid sodium-air batteries

Among alkali-air batteries, aprotic sodium-air batteries (SABs) have attracted considerable attention owing to their high theoretical specific energy (1683 W h kg-1), high Na abundance, low-cost, and environment-friendliness. However, the application of SABs is currently restricted by their limited cycling life and low energy efficiencies due to insoluble and nonconductive discharge products (NaO2 and Na2O2) generated on air electrodes. By contrast, hybrid SABs (HSABs) have resolved these daunting challenges by adopting an aqueous electrolyte cathode via a unique solid ceramic-ion-conductor-layer design separating the aprotic and aqueous electrolytes, resulting in extended cycle life. Thus, HSABs have aroused immense attention as promising next-generation energy storage systems. However, HSABs still face the key challenge of unsatisfactory cycling life that hinders their practical applications. In this review, HSAB principles are introduced, and the synthesis and rational designs of electrocatalysts based on the oxygen reduction reaction/oxygen evolution reaction from other metal-air batteries are comprehensively reviewed for the purpose of providing insight into the development of efficient air electrodes for HSABs. Furthermore, research directions of anodes, electrolytes, and air electrodes toward high-performance HSABs are proposed

A novel approach to fabricate carbon-sphere-intercalated holey graphene electrode for high-energy-density electrochemical capacitors

Desirable porous structure and huge ion-accessible surface area are crucial for rapid electronic and ionic pathway electrodes in high-performance graphene-based electrochemical capacitors. However, graphene nanosheets tend to aggregate and restack because of van der Waals interaction among graphene sheets, resulting in the loss of ion-accessible surface area and unsatisfactory electrochemical performance. To resolve this daunting challenge, a novel approach is developed for the self-assembly of holey graphene sheets intercalated with carbon spheres (H-GCS) to obtain freestanding electrodes by using a simple vacuum filtration approach and a subsequent KOH activation process. Through the introduction of carbon spheres as spacers, the restacking of reduced graphene oxide (rGO) sheets during the filtration process is mitigated efficiently. Pores on rGO sheets produced by subsequent KOH activation also provide rapid ionic diffusion kinetics and high ion-accessible electrochemical surface area, both of which favor the formation of electric double-layer capacitance. Furthermore, a higher degree of graphitization of CSs in H-GCS thin film improves the electrical conductivity of the H-GCS electrode. The H-GCS electrode exhibits 207.1 F g−1 of specific capacitance at a current density of 1 A g−1 in 6 M KOH aqueous electrolyte. Moreover, the symmetric electrochemical capacitor assembled with H-GCS electrodes and organic electrolyte is capable of delivering a maximum energy density of 29.5 Wh kg−1 and a power density of 22.6 kW kg−1