Phase-Separated Dielectric Gels Based on Christiansen Effect.

Phase separation is a trivial phenomenon but a mature strategy in materials science. The flexible materials are provided toughness and strength by phase separation, yet there are few applications in optics and electronics industry. A novel phase-separated dielectric gel (PSDG) with a strong Christiansen effect is prepared via radical polymerization using hydroxyethyl methacrylate as a monomer, 4-cyano-4'-pentylbiphenyl and tributyl citrate as mixed solvents, and polyethylene glycol as a softener. The solvent ratios and ambient conditions can efficiently change the color of PSDG which makes it strongly selective for the wavelength of transmitted light. Besides, it has a high dielectric constant (10 at 1 kHz), sensitively responding to the electric field. The phase separation degree of PSDG varies with applied electric field, which will induce its transmittance alteration accordingly. The current field sensitive PSDG provides a novel idea for "smart windows". Additionally, varying the size and shape of the electrodes can precisely control the phase separation in PSDG and also enables the function of free writing on flexible materials. Therefore, the designed PSDG has great application potential for flexible touch and interesting interactions.

Quasi-Solid-State Ion-Conducting Arrays Composite Electrolytes with Fast Ion Transport Vertical-Aligned Interfaces for All-Weather Practical Lithium-Metal Batteries.

The rapid improvement in the gel polymer electrolytes (GPEs) with high ionic conductivity brought it closer to practical applications in solid-state Li-metal batteries. The combination of solvent and polymer enables quasi-liquid fast ion transport in the GPEs. However, different ion transport capacity between solvent and polymer will cause local nonuniform Li+ distribution, leading to severe dendrite growth. In addition, the poor thermal stability of the solvent also limits the operating-temperature window of the electrolytes. Optimizing the ion transport environment and enhancing the thermal stability are two major challenges that hinder the application of GPEs. Here, a strategy by introducing ion-conducting arrays (ICA) is created by vertical-aligned montmorillonite into GPE. Rapid ion transport on the ICA was demonstrated by 6Li solid-state nuclear magnetic resonance and synchrotron X-ray diffraction, combined with computer simulations to visualize the transport process. Compared with conventional randomly dispersed fillers, ICA provides continuous interfaces to regulate the ion transport environment and enhances the tolerance of GPEs to extreme temperatures. Therefore, GPE/ICA exhibits high room-temperature ionic conductivity (1.08 mS cm-1) and long-term stable Li deposition/stripping cycles (> 1000 h). As a final proof, Li||GPE/ICA||LiFePO4 cells exhibit excellent cycle performance at wide temperature range (from 0 to 60 °C), which shows a promising path toward all-weather practical solid-state batteries.

High Lithium Salt Content PVDF-Based Solid-State Composite Polymer Electrolyte Enhanced by h-BN Nanosheets.

Due to the unique safety qualities, solid composite polymer electrolyte (SCPE) has achieved considerable attentions to fabricate high-energy-density lithium metal batteries, but its overall performance still has to be improved. Herein, a high lithium salt content poly(vinylidene fluoride) (PVDF)-based SCPE was developed, enhanced by hexagonal boron nitride (h-BN) nanosheets, presenting perfect electrochemical performance, fast ion transport, and efficient inhibition of lithium dendrite growth. The optimized SCPE (PVDF-L70-B5) could deliver high ionic conductivity (2.98×10-4  S cm-1 ), ultra-high Li+ ion transfer number (0.62), wide electrochemical stability window (5.24 V), and strong mechanical strength (3.45 MPa) at room temperature. Density functional theory calculation further confirmed that the presence of h-BN could promote the dissociation of bis(trifluoromethanesulfonyl)imide lithium (LiTFSI) and the rapid transfer of Li+ ions. As a result, the assembled symmetric Li/Li battery and asymmetric Li/LiFePO4 battery using PVDF-L70-B5 SCPEs both exhibited high reversible capacity, long-term cycle stability, and high-rate performance when cycled at 60 or 30 °C. The designed SCPEs will open up a new route to synthesize solid-state lithium batteries with high energy density and high safety.

Photoelastic Organogel with Multiple Stimuli Responses.

The photoelastic effect has many uses in mechanics today, but it is usually disregarded in flexible materials. Using 2-phenoxyethyl acrylate as a monomer and 4-cyano-4'-pentylbiphenyl (5CB) as a solvent, a multiple responsive photoelastic organogel (PO) with strong birefringence but low modulus is created. 5CB is a liquid crystal molecule that does not participate in the polymerization process and is always present as tiny molecules in the polymer. It endows the PO low modulus and high birefringence, as well as the ability to drive the birefringence using an electric field. This PO not only has high sensitivity and fast response as a photoelastic strain sensor, but also has a very sensitive response to heat, especially in the range of human body temperature. It also has a high dielectric constant and a strong correlation between the interference color and the applied electric field, allowing for easy writing and erasure of encrypted data. This unique multisignal response feature and low modulus that mimics human skin bring up new opportunities in the potential applications such as multiple information encryption, anticounterfeiting, and multifunctional wearable sensors.

An Ionically Conductive, Self-Powered and Stable Organogel for Pressure Sensing.

Gel-based ionic conductors are promising candidates for flexible electronics, serving as stretchable sensors or electrodes. However, most of them suffer from a short operating life, low conductivity and rely on an external power supply, limiting their practical application. Herein, we report a stable organogel ionic conductor with high conductivity and self-powering ability. Briefly, lithium trifluoromethanesulfonate, as a conductive salt, provides high conductivity and the poly(1,1-difluoroethylene) layers, as a self-powering system, supply stable energy output under the influence of pressure. Moreover, the proposed conductors withstand long-term and multi-cycle durability tests. The prepared auxiliary training device can withstand the impact of a basketball and detect the impact force, showing potential in passive sensing during practical applications.

Iron Selenide Microcapsules as Universal Conversion-Typed Anodes for Alkali Metal-Ion Batteries.

Rechargeable alkali metal-ion batteries (AMIBs) are receiving significant attention owing to their high energy density and low weight. The performance of AMIBs is highly dependent on the electrode materials. It is, therefore, quite crucial to explore suitable electrode materials that can fulfil the future requirements of AMIBs. Herein, a hierarchical hybrid yolk-shell structure of carbon-coated iron selenide microcapsules (FeSe2 @C-3 MCs) is prepared via facile hydrothermal reaction, carbon-coating, HCl solution etching, and then selenization treatment. When used as the conversion-typed anode materials (CTAMs) for AMIBs, the yolk-shell FeSe2 @C-3 MCs show advantages. First, the interconnected external carbon shell improves the mechanical strength of electrodes and accelerates ionic migration and electron transmission. Second, the internal electroactive FeSe2 nanoparticles effectively decrease the extent of volume expansion and avoid pulverization when compared with micro-sized solid FeSe2 . Third, the yolk-shell structure provides sufficient inner void to ensure electrolyte infiltration and mobilize the surface and near-surface reactions of electroactive FeSe2 with alkali metal ions. Consequently, the designed yolk-shell FeSe2 @C-3 MCs demonstrate enhanced electrochemical performance in lithium-ion batteries, sodium-ion batteries, and potassium-ion batteries with high specific capacities, long cyclic stability, and outstanding rate capability, presenting potential application as universal anodes for AMIBs.

Blowing Iron Chalcogenides into Two-Dimensional Flaky Hybrids with Superior Cyclability and Rate Capability for Potassium-Ion Batteries.

Chalcogenide-based anodes are receiving increasing attention for rechargeable potassium-ion batteries (PIBs) due to their high theoretical capacities. However, they usually exhibit poor electrochemical performance due to poor structural stability, low conductivity, and severe electrolyte decomposition on the reactive surface. Herein, a method analogous to "blowing bubbles with gum" is used to confine FeS2 and FeSe2 in N-doped carbon for PIB anodes with ultrahigh cyclic stability and enhanced rate capability (over 5000 cycles at 2 A g-1). Several theoretical and experimental methods are employed to understand the electrodes' performance. The density functional theory calculations showed high affinity for potassium adsorption on the FeS2 and FeSe2. The in situ XRD and ex situ TEM analysis confirmed the formation of several intermediate phases of the general formula KxFeS2. These phases have high conductivity and large interlayer distance, which promote reversible potassium insertion and facilitate the charge transfer. Also, the calculated potassium diffusion coefficient during charge/discharge further proves the enhanced kinetics. Furthermore, The FeS2@NC anode in a full cell also exhibits high cyclic stability (88% capacity retention after 120 cycles with 99.9% Coulombic efficiency). Therefore, this work provides not only an approach to overcome several challenges in PIB anodes but also a comprehensive understanding of the mechanism and kinetics of the potassium interaction with chalcogenides.

Highly Stretchable and Transparent Ionic Conductor with Novel Hydrophobicity and Extreme-Temperature Tolerance.

Highly stretchable and transparent ionic conducting materials have enabled new concepts of electronic devices denoted as iontronics, with a distinguishable working mechanism and performances from the conventional electronics. However, the existing ionic conducting materials can hardly bear the humidity and temperature change of our daily life, which has greatly hindered the development and real-world application of iontronics. Herein, we design an ion gel possessing unique traits of hydrophobicity, humidity insensitivity, wide working temperature range (exceeding 100°C, and the range covered our daily life temperature), high conductivity (10-3~10-5 S/cm), extensive stretchability, and high transparency, which is among the best-performing ionic conductors ever developed for flexible iontronics. Several ion gel-based iontronics have been demonstrated, including large-deformation sensors, electroluminescent devices, and ionic cables, which can serve for a long time under harsh conditions. The designed material opens new potential for the real-world application progress of iontronics.

Suppressing the Shuttle Effect and Dendrite Growth in Lithium-Sulfur Batteries.

Practical applications of lithium-sulfur batteries are simultaneously hindered by two serious problems occurring separately in both electrodes, namely, the shuttle effects of lithium polysulfides and the uncontrollable growth of lithium dendrites. Herein, to explore a facile integrated approach to tackle both problems as well as guarantee the efficient charge transfer, we used two-dimension hexagonal VS2 flakes as the building blocks to assemble nanotowers on the separators, forming a symmetrical double-side-modified polypropylene separator without blocking the membrane pores. Benefiting from the "sulfiphilic" and "lithiophilic" properties, high interfacial electronic conductivity, and the unique hexagonal tower-form nanostructure, the D-HVS@PP separator not only guarantees the effective suppression of the lithium polysulfide shuttle and the rapid ion/electron transfer but also realizes uniform and stable lithium nucleation and growth during cycling. Hence, just at the expense of an 11% increase in the separator weight (0.14 mg cm-2), the D-HVS@PP separator delivers an over 16 times higher initial areal capacity (8.3 mAh cm-2) than a conventional PP separator (0.5 mAh cm-2) under high sulfur-loading conditions (9.24 mg cm-2). Even when used under a low electrolyte/sulfur ratio of 4 mL g-1 and a practically relevant N/P ratio of 1.7, the D-HVS@PP separator still enabled stable cycling with a high cell-level gravimetric energy density. The potentials in broader applications (Li-S pouch battery and Li-LiFePO4 battery) and the promising commercial prospect (large-scale production and recyclability) of the developed separator are also demonstrated.

Ionic liquid assisted electrochemical coating zinc nanoparticles on carbon cloth as lithium dendrite suppressing host.

The application of lithium metal anode with high specific capacity and energy density is limited by the volume expansion and pulverization caused by dendrite growth during cycle process. We propose a composite lithium anode by immersing molten lithium on the flexible three-dimensional (3D) carbon cloth scaffold with the zinc nanoparticles. The lithiophilic zinc nanoparticles layer of framework is synthesized by fast and easy electrochemical deposition from ionic liquid avoiding high temperature, high pressure and toxic reagent. The lithium is infused into the 3D lithiophilic framework, the composite anode is obtained. The steady network structure can confine the lithium and lead to Li dendrite restraining and reducing volume change due to the low interfacial resistance and reduce the effective current density, which induced the homogeneous Li growth. Benefiting from this, the Li infused 3D carbon cloth-Zn symmetric battery exhibits a low stripping/plating overpotential (~30 mV) and can be stable over 900 h at 1 mA cm-2. The Li//LiFePO4 battery delivers higher reversible capacity (140 mAh g-1 at 2 C and 120 mAh g-1 at 5 C) and stable cycling for 1500 and 2000 cycles than bare Li.

High loading cotton cellulose-based aerogel self-standing electrode for Li-S batteries.

Lithium-sulfur (Li-S) batteries have attracted considerable attention due to their high energy density (2600 Wh kg-1). However, its commercialization is hindered seriously by the low loading and utilization rate of sulfur cathodes. Herein, we designed the cellulose-based graphene carbon composite aerogel (CCA) self-standing electrode to enhance the performance of Li-S batteries. The CCA contributes to the mass loading and utilization efficiency of sulfur, because of its unique physical structure: low density (0.018 g cm-3), large specific surface area (657.85 m2 g-1), high porosity (96%), and remarkable electrolyte adsorption (42.25 times). Compared to Al (about 49%), the CCA displayed excellent sulfur use efficiency (86%) and could reach to high area capacity of 8.60 mAh cm-2 with 9.11 mg S loading. Meanwhile, the CCA exhibits the excellent potential for pulse sensing applications due to its flexibility and superior sensitivity to electrical response signals.

3D ordered mesoporous TiO2@CMK-3 nanostructure for sodium-ion batteries with long-term and high-rate performance.

Sodium ion battery is abundant in resources and costs low, making it very competitive in the large-scale energy storage devices. The anatase TiO2 electrode material with insertion/extraction mechanism shows stable cycling performance, which is more in line with the technical requirements of large-scale energy storage batteries. To improve the electrical conductivity and stability of the TiO2 electrode materials, we have synthesized anatase TiO2 and CMK-3 composite. TiO2 nanoparticles were deposited on the surface of CMK-3 by hydrothermal reaction, and the anode material of the SIBs with 3D network structure was prepared. With the CMK-3, the structure stability, conductivity and reaction kinetics of TiO2@CMK-3 composite is improved. The electrochemical behavior is dominated by pseudocapacitance, which gives the material excellent high-rate performance. It delivers a reversible specific capacity of 186.3 mA h g-1 after 100 cycles at the current density of 50 mA g-1, 124.5 mA h g-1 after 500 long-term cycles, meanwhile it shows an outstanding rate performance, a reversible specific capacity of 105.9 mA h g-1 at 1600 mA g-1, 177.3 mA h g-1 when the current density drops to 50 mA g-1.

Highly stretchable and transparent ionic conducting elastomers.

Traditional elastomers are mostly dielectrics; existing conductive elastomers are conductive composites with electric conductors. Herein, we introduce a series of ionic conducting elastomers (ICE) by salt in polymer strategy. The ICEs possess good stretchability, transparency and ionic conductivity. Moreover, the ICEs exhibit very high stability in air, under high temperature and voltage, with excellent adhesion properties and no corrosive effects to metal electrodes. Touch sensors are fabricated using these ICEs-impedance spectra and impedance complex plane are tested and analyzed to clarify different stimulus of the touch sensors. These ICEs provide possibilities for flexible electronics and soft machines.

Ethylene glycol-mediated rapid synthesis of carbon-coated ZnFe2O4 nanoflakes with long-term and high-rate performance for lithium-ion batteries.

Carbonaceous hybrid nanocomposites with a porous flaky structure hold great promise as high-performance electrodes for lithium-ion batteries (LIBs); yet large-scale synthesis is still a challenge. In this work, we successfully develop a novel carbon hybrid structure of carbon-coated ZnFe2O4 nanoflakes (ZnFe2O4@C NFs) through a fast ethylene glycol (EG)-mediated metal alkoxide method, refluxing at 200 °C in EG and a post-calcination at 500 °C in a N2 atmosphere. The organic components in the pre-synthesized ZnFe-alkoxide precursor (ZnFe2(OCH2CH2O)4) can be transferred into an amorphous carbon layer easily surrounding the crystalline ZnFe2O4 subunits during the annealing process. The flaky morphologies of the as-prepared ZnFe2O4@C hybrids are highly dependent on the refluxing temperature. Upon increasing the refluxing temperature from 140 °C to 200 °C, the sphere-like morphology of the ZnFe2O4@C composites gradually evolves into microflowers and separate nanoflakes. When used as an anode for LIBs, the hybrid ZnFe2O4@C NFs present excellent electrochemical performance with high discharge capacity, long-term cyclic stability and superior high-rate capability. Even after 1000 cycles at 0.5 A g-1, the hybrid ZnFe2O4@C NFs still deliver a stable reversible discharge capacity of 778.6 mA h g-1.

CTAB-assisted growth of self-supported Zn2GeO4 nanosheet network on a conductive foam as a binder-free electrode for long-life lithium-ion batteries.

The Ge-based compounds show great potential as replacements for traditional graphite anode in lithium-ion batteries (LIBs). However, large volume changes and low conductivity of such materials result in a poor electrochemical cycling and rate performance. Herein, we fabricate a self-supported and three-dimensional (3D) sponge-like structure of interlinked Zn2GeO4 ultrathin nanosheets anchored vertically on a nickel foam (ZGO NSs@NF) via a simple hydrothermal process assisted by cetyltrimethyl ammonium bromide (CTAB). Such robust self-supported hybrid structures greatly improve the structural tolerance of the active materials and accommodate the volume variation that occurs during repeated electrochemical cycling. As expected, the self-supported ZGO NSs@NF composites demonstrate an excellent lithium storage with a high discharge capacity, a long cycling life, and a good rate capability when used as binder-free anodes for LIBs. A high reversible discharge capacity of 794 mA h g-1 is maintained after 500 cycles at 200 mA g-1, corresponding to 81% capacity retention of the second cycle. Further evaluation at a higher current density (2 A g-1) also delivers a reversible discharge capacity (537 mA h g-1) for this binder-free anode. This novel 3D structure of the self-supported ultrathin nanosheets on a conductive substrate, with its volume buffer effect and good interfacial contacts, can stimulate the progress of other energy-efficient technologies.

Formation of g-C3N4@Ni(OH)2 Honeycomb Nanostructure and Asymmetric Supercapacitor with High Energy and Power Density.

Nickel hydroxide (Ni(OH)2) has been regarded as a potential next-generation electrode material for supercapacitor owing to its attractive high theoretical capacitance. However, practical application of Ni(OH)2 is hindered by its lower cycling life. To overcome the inherent defects, herein we demonstrate a unique interconnected honeycomb structure of g-C3N4 and Ni(OH)2 synthesized by an environmentally friendly one-step method. In this work, g-C3N4 has excellent chemical stability and supports a perpendicular charge-transporting direction in charge-discharge process, facilitating electron transportation along that direction. The as-prepared composite exhibits higher specific capacities (1768.7 F g-1 at 7 A g-1 and 2667 F g-1 at 3 mV s-1, respectively) compared to Ni(OH)2 aggregations (968.9 F g-1 at 7 A g-1) and g-C3N4 (416.5 F g-1 at 7 A g-1), as well as better cycling performance (∼84% retentions after 4000 cycles). As asymmetric supercapacitor, g-C3N4@Ni(OH)2//graphene exhibits high capacitance (51 F g-1) and long cycle life (72% retentions after 8000 cycles). Moreover, high energy density of 43.1 Wh kg-1 and power density of 9126 W kg-1 has been achieved. This attractive performance reveals that g-C3N4@Ni(OH)2 with honeycomb architecture could find potential application as an electrode material for high-performance supercapacitors.

Tunable growth of perpendicular cobalt ferrite nanosheets on reduced graphene oxide for energy storage.

Ultrathin cobalt ferrite nanosheets have been successfully assembled on the surface of reduced graphene oxide (rGO) via only adjusting the volume ratio of ethanol and deionized (DI) water and a post calcination treatment. The perpendicular ultrathin cobalt ferrite nanosheets supported by rGO sheets (CoFe2O4 NSs@rGO) can be obtained when the volume ratio of ethanol and DI water is 10:30. Correspondingly, the hierarchical porous films covering the total rGO sheets will be formed nanosheets. When evaluated as the electrodes for lithium ion batteries (LIBs) and supercapacitors (SCs), the resultant CoFe2O4 NSs@rGO hybrids exhibit highly enhanced electrochemical performance. Even after 200 charge-discharge cycles at 400 mA g-1, the electrodes as the anode material for LIBs still exhibit a reversible discharge capacity of 835.6 mAh g-1. In addition, this electrode for SCs also exhibits specific capacitance of ca 1120 F g-1 after 3000 cycles. These superior results imply that CoFe2O4 NSs with novel hybrid structure of rGO could potentially lead to an excellent electrochemical performance for energy storage.

Free-standing ultrathin CoMn2O4 nanosheets anchored on reduced graphene oxide for high-performance supercapacitors.

Ultrathin CoMn2O4 nanosheets supported on reduced graphene oxide (rGO) are successfully synthesized through a simple co-precipitation method with a post-annealing treatment. With the assistance of citrate, the free-standing CoMn2O4 ultrathin nanosheets can form porous overlays on both sides of the rGO sheets. Such a novel hybrid nanostructure can effectively promote charge transport and accommodate volume variation upon prolonged charge/discharge cycling. When evaluated as a promising electrode for supercapacitors in a 6 M KOH solution electrolyte, the hybrid nanocomposites demonstrate highly enhanced capacitance and excellent cycling stability.

Growth of Ultrathin ZnCo2O4 Nanosheets on Reduced Graphene Oxide with Enhanced Lithium Storage Properties.

The growth of ultrathin ZnCo2O4 nanosheets on reduced graphene oxide (denoted as rGO/ZnCo2O4) via a facile low-temperature solution method combined with a subsequent annealing treatment is reported. With the assistance of citrate, interconnected ZnCo2O4 nanosheets can assemble into hierarchically porous overlays on both sides of rGO sheets. Such a hybrid nanostructure would effectively faciliate the charge transport and accommodate volume variation upon prolonged charge/discharge cycling for reversible lithium storage. As a result, the rGO/ZnCo2O4 nanocomposite manifests a very stable high reversible capacity of around 960 mAh g-1 over 100 cycles at a low current density of 90 mA g-1 and excellent rate capability.

Hierarchical NiCo2 O4 nanosheets grown on Ni nanofoam as high-performance electrodes for supercapacitors.

A high-performance electrode for supercapacitors is designed and synthesized by growing electroactive NiCo2 O4 nanosheets on conductive Ni nanofoam. Because of the structural advantages, the as-prepared Ni@NiCo2 O4 hybrid nanostructure exhibits significantly improved electrochemical performance with high capacitance, excellent rate capability, and good cycling stability.