Yun Song

Pseudocapacitance-tuned high-rate and long-term cyclability of NiCo2S4 hexagonal nanosheets prepared by vapor transformation for lithium storage

The high conductivity of bimetallic thiospinel NiCo2S4 endows energy storage devices with very fascinating performance. However, the unsatisfactory rate capability and long-term cyclability of this material series significantly limit their large-scale practical applications such as in electric vehicles and hybrid electric vehicles. Herein, we successfully synthesized NiCo2S4 hexagonal nanosheets with a large lateral dimension of ∼1.35 μm and a thickness of ∼30 nm through a vapor transformation method. The dynamic transformation process of the NiCo2S4 polycrystalline nanosheets from NiCo-hydroxide has been revealed in detail. Originating from their two-dimensional thin-sheet structure with a high aspect ratio, the induced extrinsic capacitive contribution as high as 91% makes them an ideal candidate for high-capacity and high-rate lithium-ion anodes. The NiCo2S4 nanosheets deliver a reversible capacity of 607 mA h g−1 upon 800 cycles at a current density of 2 A g−1. This outstanding long cycle performance sheds light on the structural design of electrode materials for high-rate lithium-ion batteries.

Bottom-up Approach Design, Band Structure, and Lithium Storage Properties of Atomically Thin γ-FeOOH Nanosheets

As a novel class of soft matter, two-dimensional (2D) atomic nanosheet-like crystals have attracted much attention for energy storage devices due to the fact that nearly all of the atoms can be exposed to the electrolyte and involved in redox reactions. Herein, atomically thin γ-FeOOH nanosheets with a thickness of ∼1.5 nm are synthesized in a high yield, and the band and electronic structures of the γ-FeOOH nanosheet are revealed using density-functional theory calculations for the first time. The rationally designed γ-FeOOH@rGO composites with a heterostacking structure are used as an anode material for lithium-ion batteries (LIBs). A high reversible capacity over 850 mAh g(-1) after 100 cycles at 200 mA g(-1) is obtained with excellent rate capability. The remarkable performance is attributed to the ultrathin nature of γ-FeOOH nanosheets and 2D heterostacking structure, which provide the minimized Li(+) diffusion length and buffer zone for volume change. Further investigation on the Li storage electrochemical mechanism of γ-FeOOH@rGO indicates that the charge-discharge processes include both conversion reaction and capacitive behavior. This synergistic effect of conversion reaction and capacitive behavior originating from 2D heterostacking structure casts new light on the development of high-energy anode materials.

Carbon nanomaterial-assisted morphological tuning for thermodynamic and kinetic destabilization in sodium alanates

In the present work, we develop an effective strategy, i.e., carbon nanomaterial-assisted morphological tuning, for both thermodynamic and kinetic destabilization in complex hydrides based on the interaction between the complex anion and the carbon matrix. The NaAlH4/carbon nanomaterials of graphene nanosheets (GNs), fullerene (C60) and mesoporous carbon (MC) were selected as model systems for illustrating the positive effect of carbon nanomaterial-assisted morphological tuning. It is demonstrated that through the dissolution–recrystallization process, the morphologies of NaAlH4 can be altered from the scale-like continuous structure for the GN-assisted sample, to flower-like structures with diameters ranging from 5 to 10 μm for the C60-assisted sample and to uniform particles with an average diameter of about 2 μm for the MC-assisted sample. Correspondingly, the onset temperature for dehydrogenation of NaAlH4 is reduced to about 188, 185 and 160 °C for the samples assisted with GNs, C60 and MC, respectively, much lower than 210 °C for the pristine sample. A remarkable reduction in activation energy for three-step dehydrogenation is also obtained in NaAlH4/carbon nanomaterial composites relative to the pristine sample, and the improved efficiency of carbon nanomaterials for kinetics is found to be in the order of MC > C60 > GNs. These positive improvements can be attributed to both the particle refinement and interaction between NaAlH4 and the carbon nanomaterial that are in intimate contact with each other, which are not only evidenced by FE-SEM observation, but also supported by 27Al solid-state NMR characterization.

In-situ Growth of Layered Bimetallic ZnCo Hydroxide Nanosheets for High-Performance All-Solid-State Pseudocapacitor

Two-dimensional (2D) hydroxide nanosheets can exhibit exceptional electrochemical performance owing to their shortened ion diffusion distances, abundant active sites, and various valence states. Herein, we report ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets (thickness ∼30 nm) which crystallize in a layered structure and exhibit a high specific capacitance of 3946.5 F g-1 at 3 A g-1 for an electrochemical pseudocapacitor. ZnCo1.5(OH)4.5Cl0.5·0.45H2O was synthesized by a homogeneous precipitation method and spontaneously crystallized into 2D nanosheets in well-defined hexagonal morphology with crystal structure revealed by synchrotron X-ray powder diffraction data analysis. In situ growth of ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheet arrays on conductive Ni foam substrate was successfully realized. Asymmetric supercapacitors based on ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets @Ni foam// PVA, KOH//reduced graphene oxide exhibits a high energy density of 114.8 Wh kg-1 at an average power density of 643.8 W kg-1, which surpasses most of the reported all-solid-state supercapacitors based on carbonaceous materials, transition metal oxides/hydroxides, and MXenes. Furthermore, a supercapacitor constructed from ZnCo1.5(OH)4.5Cl0.5·0.45H2O nanosheets@PET substrate shows excellent flexibility and mechanical stability. This study provides layered bimetallic hydroxide nanosheets as promising electroactive materials for flexible, solid-state energy storage devices, presenting the best reported performance to date.

Embedding ZnSe nanodots in nitrogen-doped hollow carbon architectures for superior lithium storage

Transition metal chalcogenides represent a class of the most promising alternative electrode materials for high-performance lithium-ion batteries (LIBs) owing to their high theoretical capacities. However, they suffer from large volume expansion, particle agglomeration, and low conductivity during charge/discharge processes, leading to unsatisfactory energy storage performance. In order to address these issues, we rationally designed three-dimensional (3D) hybrid composites consisting of ZnSe nanodots uniformly confined within a N-doped porous carbon network (ZnSe ND@N-PC) obtained via a convenient pyrolysis process. When used as anodes for LIBs, the composites exhibited outstanding electrochemical performance, with a high reversible capacity (1,134 mA·h·g−1 at a current density of 600 mA·g−1 after 500 cycles) and excellent rate capability (696 and 474 mA·h·g−1 at current densities of 6.4 and 12.8 A·g−1, respectively). The significantly improved lithium storage performance can be attributed to the 3D architecture of the hybrid composites, which not only mitigated the internal mechanical stress induced by the volume change and formed a 3D conductive network during cycling, but also provided a large reactive area and reduced the lithium diffusion distance. The strategy reported here may open a new avenue for the design of other multifunctional composites towards high-performance energy storage devices.

Charge Transfer in Ultrafine LDH Nanosheets/Graphene Interface with Superior Capacitive Energy Storage Performance

Two-dimensional LDH nanosheets recently have generated considerable interest in various promising applications because of their intriguing properties. Herein, we report a facile in situ nucleation strategy toward in situ decorating monodispersed Ni-Fe LDH ultrafine nanosheets (UNs) on graphene oxide template based on the precise control and manipulation of LDH UNs anchored, nucleated, grown, and crystallized. Anion-exchange behavior was observed in this Ni-Fe LDH UNs@rGO composite. The Ni-Fe LDH UNs@rGO electrodes displayed a significantly enhanced specific capacitance (2715F g-1 at 3 A g-1) and energy density (82.3 Wh kg-1 at 661 W kg-1), which exceeds the energy densities of most previously reported nickel iron oxide/hydroxides. Moreover, the asymmetric supercapacitor, with the Ni-Fe LDH UNs @rGO composite as the positive electrode material and reduced graphene oxide (rGO) as the negative electrode material, exhibited a high energy density (120 Wh kg -1) at an average power density of 1.3 kW kg -1. A charge transfer from LDH layer to graphene layer, which means a built in electric field directed from LDH to graphene can be established by DFT calculations, which can significantly accelerate reaction kinetics and effectively optimize the capacitive energy storage performance.

Fast hydrogen-induced optical and electrical transitions of Mg and Mg-Ni films with amorphous structure

Amorphous Mg and MgNix (x = 0.03–0.30) films were prepared and their optical and electrical transitions upon hydrogen loading/unloading at room temperature were investigated. The results show that amorphous films have faster optical and electrical transitions than corresponding crystalline ones. Amorphous structure greatly facilitates hydrogen diffusion, resulting in not only preventing the formation of blocking hydrides layers but also changing the rate-controlling step from hydrogen diffusion for crystalline film to the reaction between hydrogen and Mg and/or Mg-Ni phases. We envision that amorphization is generally applicable to improve hydrogen-induced response characteristics of switchable mirror thin films.

Facile self-assembly of light metal borohydrides with controllable nanostructures

Herein, a self-assembly strategy for the realization of diverse light metal complex borohydride nanoparticles (NPs) with sphere, polygon and hollow geometries is presented. Under ambient conditions, the particle sizes of the LiBH4 NPs formed can easily be controlled by varying the concentration which enables hydrogen gas release at ∼72 °C.

Rapid Amorphization in Metastable CoSeO3·H2O Nanosheets for Ultrafast Lithiation Kinetics

The realization of high-performance anode materials with high capacity at fast lithiation kinetics and excellent cycle stability remains a significant but critical challenge for high-power applications such as electric vehicles. Two-dimensional nanostructures have attracted considerable research interest in electrochemical energy storage devices owing to their intriguing surface effect and significantly decreased ion-diffusion pathway. Here we describe rationally designed metastable CoSeO3·H2O nanosheets synthesized by a facile hydrothermal method for use as a Li ion battery anode. This crystalline nanosheet can be steadily converted into amorphous phase at the beginning of the first Li+ discharge cycling, leading to ultrahigh reversible capacities of 1100 and 515 mAh g-1 after 1000 cycles at a high rate of 3 and 10 A g-1, respectively. The as-obtained amorphous structure experiences an isotropic stress, which can significantly reduce the risk of fracture during electrochemical cycling. Our study offers a precious opportunity to reveal the ultrafast lithiation kinetics associated with the rapid amorphization mechanism in layered cobalt selenide nanosheets.

CuGaS2 nanoplates: a robust and self-healing anode for Li/Na ion batteries in a wide temperature range of 268–318 K

The practical application of batteries in electric vehicles (EVs) and hybrid electric vehicles (HEVs) is hindered by the narrow operating temperature range due to the degradation of the solid–electrolyte interface (SEI) layer at high temperature and poor ion/electron diffusion kinetics at low temperature. Herein, we firstly report CuGaS2 hexagonal nanoplates as a novel and robust anode material working in a wide temperature range. CuGaS2 nanoplates with a lateral size of 2–3 μm and thickness of 180–200 nm have been successfully synthesized by a vapor thermal transformation from the oxide counterpart CuGaO2. The thermal workability of the as-synthesized CuGaS2 benefits from a synergistic effect including the high conductivity of copper and the self-healing nature of liquid metal gallium. Room temperature CuGaS2 as a lithium ion battery anode electrode exhibits a high reversible capacity over 521 mA h g−1 after 600 cycles at a high current density of 5 A g−1. Furthermore, as the temperature is lifted to 318 K, the CuGaS2 electrode exhibits a stable and reversible capacity over 784 mA h g−1 at a high current density of 0.5 A g−1; even at the low temperature of 268 K, a reversible capacity over 407 mA h g−1 can be realized, which is much superior to that of the commercial graphite anode.