Intercalation Pseudocapacitance in Ultrathin VOPO₄ Nanosheets: Toward High-Rate Alkali-Ion-Based Electrochemical Energy Storage

There is a growing need for energy storage devices in numerous applications where a large amount of energy needs to be either stored or delivered quickly. The present paper details the study of alkali-ion intercalation pseudocapacitance in ultrathin VOPO₄ nanosheets, which hold promise in high-rate alkali-ion based electrochemical energy storage. Starting from bulk VOPO₄·2H₂O chunks, VOPO₄ nanosheets were obtained through simple ultrasonication in 2-propanol. These nanosheets as the cathode exhibit a specific capacity of 154 and 136 mAh/g (close to theoretical value 166 mAh/g) for lithium and sodium storage devices at 0.1 C and 100 and ∼70 mAh/g at 5 C, demonstrating their high rate capability. Moreover, the capacity retention is maintained at 90% for lithium ion storage and 73% for sodium ion storage after 500 cycles, showing their reasonable stability. The demonstrated alkali-ion intercalation pseudocapacitance represents a promising direction for developing battery materials with promising high rate capability

Single-Crystalline LiFePO₄ Nanosheets for High-Rate Li-Ion Batteries

The lithiation/delithiation in LiFePO₄ is highly anisotropic with lithium-ion diffusion being mainly confined to channels along the <i>b</i>-axis. Controlling the orientation of LiFePO₄ crystals therefore plays an important role for efficient mass transport within this material. We report here the preparation of single crystalline LiFePO₄ nanosheets with a large percentage of highly oriented {010} facets, which provide the highest pore density for lithium-ion insertion/extraction. The LiFePO₄ nanosheets show a high specific capacity at low charge/discharge rates and retain significant capacities at high C-rates, which may benefit the development of lithium batteries with both favorable energy and power density

Layer-by-Layer Assembly of Two-Dimensional Colloidal Cu₂Se Nanoplates and Their Layer-Dependent Conductivity

Colloidal assembly is one of the highly active areas in nanoscale science and technology as it plays an important role in organizing nanoscale building blocks into hierarchical and functional systems for practical applications. Here, we report large scale assemblies of two-dimensional (2D) Cu₂Se nanoplates synthesized by a microwave-assisted polyol method with careful phase control. Thin films of Cu₂Se were obtained via the Langmuir–Blodgett (LB) method in a layer-by-layer manner. Interestingly, despite the decrease in volume fraction of Cu₂Se as layer number increases, the films show an increasing trend in conductivity. We propose a “layer-dependent conducting-bridge” (LDCB) model considering density of conducting points and possible defects, and the simulated trend of conductivity exhibits a corresponding match with experimental measurements. Our study serves as an important extension of colloidal assembly in 2D nanostructures, and the proposed conductivity model provides insights into the understanding of electron transport inside 2D ordered matrix

Ultrathin Two-Dimensional MnO₂/Graphene Hybrid Nanostructures for High-Performance, Flexible Planar Supercapacitors

Planar supercapacitors have recently attracted much attention owing to their unique and advantageous design for 2D nanomaterials based energy storage devices. However, improving the electrochemical performance of planar supercapacitors still remains a great challenge. Here we report for the first time a novel, high-performance in-plane supercapacitor based on hybrid nanostructures of quasi-2D ultrathin MnO₂/graphene nanosheets. Specifically, the planar structures based on the δ-MnO₂ nanosheets integrated on graphene sheets not only introduce more electrochemically active surfaces for absorption/desorption of electrolyte ions, but also bring additional interfaces at the hybridized interlayer areas to facilitate charge transport during charging/discharging processes. The unique structural design for planar supercapacitors enables great performance enhancements compared to graphene-only devices, exhibiting high specific capacitances of 267 F/g at current density of 0.2 A/g and 208 F/g at 10 A/g and excellent rate capability and cycling stability with capacitance retention of 92% after 7000 charge/discharge cycles. Moreover, the high planar malleability of planar supercapacitors makes possible superior flexibility and robust cyclability, yielding capacitance retention over 90% after 1000 times of folding/unfolding. Ultrathin 2D nanomaterials represent a promising material platform to realize highly flexible planar energy storage devices as the power back-ups for stretchable/flexible electronic devices

Biobased Nano Porous Active Carbon Fibers for High-Performance Supercapacitors

Activated carbon fibers (ACFs) with different pore structure have been prepared from wood sawdust using the KOH activation method. A study was conducted to examine the influence of the activation parameters (temperature, alkali/carbon ratio, and time) on the morphology and structure of the as-prepared ACFs developed in the process of pore generation and evolution. Activation temperature was very essential for the formation of utramicropores (<0.6 nm), which greatly contributed to the electric double layer capacitance. The significance of metallic potassium vapor evolved when the temperature was above 800 °C, since the generation of 0.8- and 1.1 nm micropores cannot be ignored. When the the KOH/fiber ratio was increased and the activation time was prolonged, to some extent, the micropores were enlarged to small mesopores within 2–5 nm. The sample with the optimal condition exhibited the highest specific capacitance (225 F g^–1 at a current density of 0.5 A g^–1). Its ability to retain capacitance corresponding to 10 A g^–1 and 6 M KOH was 85.3%, demonstrating a good rate capability. With 10 000 charge–discharge cycles at 3 A g^–1, the supercapacitor kept 94.2% capacity, showing outstanding electrochemical performance as promising electrode material

Chemically Integrated Two-Dimensional Hybrid Zinc Manganate/Graphene Nanosheets with Enhanced Lithium Storage Capability

Hybrid inorganic/graphene two-dimensional (2D) nanostructures can offer vastly open large surface areas for ion transport and storage and enhanced electron transport, representing a promising material platform for next-generation energy storage. Here we report chemically integrated hybrid ZnMn₂O₄/graphene nanosheets synthesized <i>via</i> a facile two-step method for greatly enhanced lithium storage capability. The hybrid 2D nanosheets are composed of ultrafine ZnMn₂O₄ nanocrystals with a mean diameter of ∼4 nm attached to and well dispersed on the surface of reduced graphene oxide sheets. The hybrid nanosheets based anode offers a high capacity of ∼800 mAh g^–1 at a current rate of 500 mA g^–1, excellent rate capability, and long-term cyclability with reversible capacity of ∼650 mAh g^–1 over 1500 cycles at a current density of 2000 mA g^–1. Moreover, when tested in a temperature range of ∼0–60 °C, the designed anode can maintain high discharge capacities from 570 to 820 mAh g^–1

Enabling Enhanced Cycling Stability of a LiNi_0.8Co_0.15Al_0.05O₂ Cathode by Constructing a Ti-Rich Surface

Herein, we construct a Ti-rich interface of a LiNi0.8Co0.15Al0.05O2 (NCA) secondary particle using titanium nitride (TiN) nanopowders as a dopant to reduce interfacial reaction. Results show that Ti ions integrate into the layered lattice during the dissociation of Ti–N and was enriched within the surface layer. The solid Ti–O bonding effectively enhances the interface stability and reduces lattice change toward an improved cycle stability. As a result, continuous growth of CEI film and dissolution of transition metal elements were depressed. Both thinner cathode–electrolyte interphases (CEI) and phase transition layers form on the surface of particles after a long cycle. The Ti-doping NCA cathode (NCATiN) provides a better capacity retention of 90.9% over 200 cycles

Cyanogel-Enabled Homogeneous Sb–Ni–C Ternary Framework Electrodes for Enhanced Sodium Storage

Antimony (Sb) represents an important high-capacity anode material for advanced sodium ion batteries, but its practical utilization has been primarily hampered by huge volume expansion-induced poor cycling life. The co-incorporation of transition-metal (M = Ni, Cu, Fe, <i>etc.</i>) and carbon components can synergistically buffer the volume change of the Sb component; however, these Sb–M–C ternary anodes often suffer from uneven distribution of Sb, M, and C components. Herein, we propose a general nanostructured gel-enabled methodology to synthesize homogeneous Sb–M–C ternary anodes for fully realizing the synergestic effects from M/C dual matrices. A cyano-bridged Sb­(III)–Ni­(II) coordination polymer gel (Sb–Ni cyanogel) has been synthesized and directly reduced to an Sb–Ni alloy framework (Sb–Ni framework). Moreover, graphene oxide (GO) can be <i>in situ</i> immobilized within the cyanogel framework, and after reduction, reduced graphene oxide (rGO) is uniformly distributed within the alloy framework, yielding a homogeneous rGO@Sb–Ni ternary framework. The rGO@Sb–Ni framework with optimal rGO content manifests a high reversible capacity of ∼468 mA h g^–1 at 1 A g^–1 and stable cycle life at 5 A g^–1 (∼210 mA h g^–1 after 500 cycles). The proposed cyanogel-enabled methodology may be extended to synthesize other homogeneous ternary framework materials for efficient energy storage and electrocatalysis

General Facet-Controlled Synthesis of Single-Crystalline {010}-Oriented LiMPO₄ (M = Mn, Fe, Co) Nanosheets

Facet-controlled synthesis of phospho-olivine (LiMPO₄, M = Mn, Fe, Co) cathode materials is of particular interest to manipulate their electrochemical properties because of their anisotropic ionic transport behavior. This study provides a general facet-controlled synthesis of single-crystalline LiMPO₄ (M = Mn, Fe, Co) nanosheets with significantly large exposure of (010)-facets, which has not been readily achieved by conventional solution-based coprecipitation or solid-reaction methods. The as-obtained nanosheets show controllable thickness with the thinnest thickness down to 15–20 nm and lateral dimension up to ∼5 μm. Due to the shortened lithium ion diffusion pathway and high ratio of active surface enabled by the thin thickness, the as-prepared LiFePO₄ nanosheets, as a model material, demonstrate greatly improved rate capability and cycling stability, with a reversible capacity of ∼80 mA h g^–1 at a current rate of 30 C and a stable capacity retention of ∼93% after 500 cycles at a current rate of 5 C. Further electrochemical analysis reveals an enhanced interfacial lithium ion diffusion of the nanosheets, suggesting that facet-controlled 2D LiMPO₄ nanosheets are a promising material platform for next-generation high-rate lithium-ion batteries

Metallic Few-Layered VS₂ Ultrathin Nanosheets: High Two-Dimensional Conductivity for In-Plane Supercapacitors

With the rapid development of portable electronics, such as e-paper and other flexible devices, practical power sources with ultrathin geometries become an important prerequisite, in which supercapacitors with in-plane configurations are recently emerging as a favorable and competitive candidate. As is known, electrode materials with two-dimensional (2D) permeable channels, high-conductivity structural scaffolds, and high specific surface areas are the indispensible requirements for the development of in-plane supercapacitors with superior performance, while it is difficult for the presently available inorganic materials to make the best in all aspects. In this sense, vanadium disulfide (VS₂) presents an ideal material platform due to its synergic properties of metallic nature and exfoliative characteristic brought by the conducting S–V–S layers stacked up by weak van der Waals interlayer interactions, offering great potential as high-performance in-plane supercapacitor electrodes. Herein, we developed a unique ammonia-assisted strategy to exfoliate bulk VS₂ flakes into ultrathin VS₂ nanosheets stacked with less than five S–V–S single layers, representing a brand new two-dimensional material having metallic behavior aside from graphene. Moreover, highly conductive VS₂ thin films were successfully assembled for constructing the electrodes of in-plane supercapacitors. As is expected, a specific capacitance of 4760 μF/cm² was realized here in a 150 nm in-plane configuration, of which no obvious degradation was observed even after 1000 charge/discharge cycles, offering as a new in-plane supercapacitor with high performance based on quasi-two-dimensional materials