Multilayer Approach for Advanced Hybrid Lithium Battery

Conventional intercalated rechargeable batteries have shown their capacity limit, and the development of an alternative battery system with higher capacity is strongly needed for sustainable electrical vehicles and hand-held devices. Herein, we introduce a feasible and scalable multilayer approach to fabricate a promising hybrid lithium battery with superior capacity and multivoltage plateaus. A sulfur-rich electrode (90 wt % S) is covered by a dual layer of graphite/Li₄Ti₅O₁₂, where the active materials S and Li₄Ti₅O₁₂ can both take part in redox reactions and thus deliver a high capacity of 572 mAh g_cathode^–1 (<i>vs</i> the total mass of electrode) or 1866 mAh g_s^–1 (<i>vs</i> the mass of sulfur) at 0.1C (with the definition of 1C = 1675 mA g_s^–1). The battery shows unique voltage platforms at 2.35 and 2.1 V, contributed from S, and 1.55 V from Li₄Ti₅O₁₂. A high rate capability of 566 mAh g_cathode^–1 at 0.25C and 376 mAh g_cathode^–1 at 1C with durable cycle ability over 100 cycles can be achieved. Operando Raman and electron microscope analysis confirm that the graphite/Li₄Ti₅O₁₂ layer slows the dissolution/migration of polysulfides, thereby giving rise to a higher sulfur utilization and a slower capacity decay. This advanced hybrid battery with a multilayer concept for marrying different voltage plateaus from various electrode materials opens a way of providing tunable capacity and multiple voltage platforms for energy device applications

Band Gap Tuning of Graphene by Adsorption of Aromatic Molecules

The effects of adsorbing simple aromatic molecules on the electronic structure of graphene were systematically examined by first-principles calculations. Adsorptions of different aromatic molecules borazine (B₃N₃H₆), triazine (C₃N₃H₃), and benzene (C₆H₆) on graphene have been investigated, and we found that molecular adsorptions often lead to band gap opening. While the magnitude of band gap depends on the adsorption site, in the case of C₃N₃H₃, the value of the band gap is found to be up to 62.9 meV under local density approximationwhich is known to underestimate the gap. A couple of general trends were noted: (1) heterocyclic molecules are more effective than moncyclic ones and (2) the most stable configuration of a given molecule always leads to the largest band gap. We further analyzed the charge redistribution patterns at different adsorption sites and found that they play an important role in controling the on/off switching of the gapthat is, the energy gap is opened if the charge redistributes to between the C–C bond when the molecule is adsorbing on graphene. These trends suggest that the different ionic ability of two atoms in heterocyclic molecules can be used to control the charge redistribution on graphene and thus to tune the gap using different adsorption conditions

Unraveling Spatially Heterogeneous Ultrafast Carrier Dynamics of Single-Layer WSe₂ by Femtosecond Time-Resolved Photoemission Electron Microscopy

Studies of the ultrafast carrier dynamics of transition metal dichalcogenides have employed spatially averaged measurements, which obfuscate the rich variety of dynamics that originate from the structural heterogeneity of these materials. Here, we employ femtosecond time-resolved photoemission electron microscopy (TR-PEEM) with sub-80 nm spatial resolution to image the ultrafast subpicosecond to picosecond carrier dynamics of monolayer tungsten diselenide (WSe₂). The dynamics observed following 2.41 eV pump and 3.61 eV probe occurs on two distinct time scales. The 0.1 ps process is assigned to electron cooling via intervalley scattering, whereas the picosecond dynamics is attributed to exciton–exciton annihilation. The 70 fs decay dynamics observed at negative time delay reflects electronic relaxation from the Γ point. Analysis of the TR-PEEM data furnishes the spatial distributions of the various time constants within a single WSe₂ flake. The spatial heterogeneity of the lifetime maps is consistent with increased disorder along the edges of the flake and the presence of nanoscale charge puddles in the interior. Our results indicate the need to go beyond spatially averaged time-resolved measurements to understand the influence of structural heterogeneities on the elementary carrier dynamics of two-dimensional materials

Nanoscale Surface Photovoltage Mapping of 2D Materials and Heterostructures by Illuminated Kelvin Probe Force Microscopy

Nanomaterials are interesting for a variety of applications, such as optoelectronics and photovoltaics. However, they often have spatial heterogeneity, i.e., composition change or physical change in the topography or structure, which can lead to varying properties that would influence their applications. New techniques must be developed to understand and correlate spatial heterogeneity with changes in electronic properties. Here we highlight the technique of surface photovoltage Kelvin probe force microscopy (SPV-KFM), which is a modified version of noncontact atomic force microscopy capable of imaging not only the topography and surface potential but also the surface photovoltage on the nanoscale. We demonstrate its utility in probing monolayer WSe₂–MoS₂ lateral heterostructures, which form an ultrathin p–n junction promising for photovoltaic and optoelectronic applications. We show surface photovoltage maps highlighting the different photoresponse of the two material regions as a result of the effective charge separation across this junction. Additionally, we study the variations between different heterostructure flakes and emphasize the importance of controlling the synthesis and transfer of these materials to obtain consistent properties and measurements

Redox Species-Based Electrolytes for Advanced Rechargeable Lithium Ion Batteries

Seeking high-capacity cathodes has become an intensive effort in lithium ion battery research; however, the low energy density still remains a major issue for sustainable handheld devices and vehicles. Herein, we present a new strategy of integrating a redox species-based electrolyte in batteries to boost their performance. Taking the olivine LiFePO₄-based battery as an example, the incorporation of redox species (i.e., polysulfide of Li₂S₈) in the electrolyte results in much lower polarization and superior stability, where the dissociated Li⁺/S_<i>x</i>^2– can significantly speed up the lithium diffusion. More importantly, the presence of the S₈^2–/S^2– redox reaction further contributes extra capacity, making a completely new LiFePO₄/Li₂S_<i>x</i> hybrid battery with a high energy density of 1124 Wh kg_cathode^–1 and a capacity of 442 mAh g_cathode^–1. The marriage of appropriate redox species in an electrolyte for a rechargeable battery is an efficient and scalable approach for obtaining higher energy density storage devices

Highly Flexible MoS₂ Thin-Film Transistors with Ion Gel Dielectrics

Molybdenum disulfide (MoS₂) thin-film transistors were fabricated with ion gel gate dielectrics. These thin-film transistors exhibited excellent band transport with a low threshold voltage (<1 V), high mobility (12.5 cm²/(V·s)) and a high on/off current ratio (10⁵). Furthermore, the MoS₂ transistors exhibited remarkably high mechanical flexibility, and no degradation in the electrical characteristics was observed when they were significantly bent to a curvature radius of 0.75 mm. The superior electrical performance and excellent pliability of MoS₂ films make them suitable for use in large-area flexible electronics

Thermal Redox Desalination of Seawater Driven by Temperature Difference

Millions of people suffer from the crisis of lacking sufficient freshwater, particularly for the regions or occasions without proper electricity supply. Therefore, desalination of seawater with low power and renewable energy has become an important issue. Here, we proposed an electrolyte-based thermal redox desalination (ETRD) device powered by temperature difference, where two electrodes are in contact with circulating [Fe(CN)6]3–/4‑ redox electrolytes at different temperatures sandwiching two salt streams in between. A single ETRD device operating at temperature difference of 60 K, with a large Seebeck coefficient of 1.82 mV K–1 and a high potential difference of 113 mV, enables a maximum power density of 188.8 mW m–2 released and salt removal rate of 9.77 μg cm–2 min–1 with reusability and stability. Besides, seawater can be spontaneously desalinated to freshwater standard while generating electricity. This work shows that ETRD device can realize triple functions including low-grade heat utilization, seawater desalination, and energy supply

Thermal Redox Desalination of Seawater Driven by Temperature Difference

Millions of people suffer from the crisis of lacking sufficient freshwater, particularly for the regions or occasions without proper electricity supply. Therefore, desalination of seawater with low power and renewable energy has become an important issue. Here, we proposed an electrolyte-based thermal redox desalination (ETRD) device powered by temperature difference, where two electrodes are in contact with circulating [Fe(CN)6]3–/4‑ redox electrolytes at different temperatures sandwiching two salt streams in between. A single ETRD device operating at temperature difference of 60 K, with a large Seebeck coefficient of 1.82 mV K–1 and a high potential difference of 113 mV, enables a maximum power density of 188.8 mW m–2 released and salt removal rate of 9.77 μg cm–2 min–1 with reusability and stability. Besides, seawater can be spontaneously desalinated to freshwater standard while generating electricity. This work shows that ETRD device can realize triple functions including low-grade heat utilization, seawater desalination, and energy supply

Scalable Approach To Construct Free-Standing and Flexible Carbon Networks for Lithium–Sulfur Battery

Reconstructing carbon nanomaterials (e.g., fullerene, carbon nanotubes (CNTs), and graphene) to multidimensional networks with hierarchical structure is a critical step in exploring their applications. Herein, a sacrificial template method by casting strategy is developed to prepare highly flexible and free-standing carbon film consisting of CNTs, graphene, or both. The scalable size, ultralight and binder-free characteristics, as well as the tunable process/property are promising for their large-scale applications, such as utilizing as interlayers in lithium–sulfur battery. The capability of holding polysulfides (i.e., suppressing the sulfur diffusion) for the networks made from CNTs, graphene, or their mixture is pronounced, among which CNTs are the best. The diffusion process of polysulfides can be visualized in a specially designed glass tube battery. X-ray photoelectron spectroscopy analysis of discharged electrodes was performed to characterize the species in electrodes. A detailed analysis of lithium diffusion constant, electrochemical impedance, and elementary distribution of sulfur in electrodes has been performed to further illustrate the differences of different carbon interlayers for Li–S batteries. The proposed simple and enlargeable production of carbon-based networks may facilitate their applications in battery industry even as a flexible cathode directly. The versatile and reconstructive strategy is extendable to prepare other flexible films and/or membranes for wider applications

Layer-by-Layer Graphene/TCNQ Stacked Films as Conducting Anodes for Organic Solar Cells

Large-area graphene grown by chemical vapor deposition (CVD) is a promising candidate for transparent conducting electrode applications in flexible optoelectronic devices such as light-emitting diodes or organic solar cells. However, the power conversion efficiency (PCE) of the polymer photovoltaic devices using a pristine CVD graphene anode is still not appealing due to its much lower conductivity than that of conventional indium tin oxide. We report a layer-by-layer molecular doping process on graphene for forming sandwiched graphene/tetracyanoquinodimethane (TCNQ)/graphene stacked films for polymer solar cell anodes, where the TCNQ molecules (as p-dopants) were securely embedded between two graphene layers. Poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester (P3HT/PCBM) bulk heterojunction polymer solar cells based on these multilayered graphene/TCNQ anodes are fabricated and characterized. The P3HT/PCBM device with an anode structure composed of two TCNQ layers sandwiched by three CVD graphene layers shows optimum PCE (∼2.58%), which makes the proposed anode film quite attractive for next-generation flexible devices demanding high conductivity and transparency