Architecture and Performance of the Novel Sulfur Host Material Based on Ti2O3 Microspheres for Lithium-Sulfur Batteries

Porous TiO2-x/C Nanofibers with Axially Aligned Tunnel Pores as Effective Sulfur Hosts for Stabilized Lithium-Sulfur Batteries

Hierarchically porous TiO_2-x/C nanofibers (NFs) with axially aligned cylindrical tunnel pore channels were synthesized as a sulfur (S) host for lithium-sulfur batteries (LSBs) by a microemulsion electrospinning method. We explored a synergistic chemical trapping reinforced by coordinatively unsaturated Ti³⁺ nuclei with oxygen deficiency (or more broadly via polar O-Ti-O units) in combination with physical trapping in both narrow pores (<5 nm) and larger ordered pore tunnels (20-100 nm) separated by thin walls to allow for a large volume of active material and rapid diffusion within the channels while effectively blocking out the diffusion of soluble lithium polysulfides. Due to this unique architecture and enhanced conductivity, the prepared materials enabled a high S loading (∼72 wt %) and significantly reduced the shuttle effect. Hence, the composite TiO_2-x/C@S cathodes exhibited a high utilization of active materials, excellent rate performance, and promising cycling stability (retention of up to ∼1010 mAh g^-1 after 150 cycles for the aerial capacity of 1.5 mAh cm^-2, with very stable performance even for the high S loading of 2.5 mg cm^-2). By designing control nanomaterials that lack either the pore tunnels or the desired chemical compositions, we elucidated the importance of the synergistic effect of both factors. This work demonstrates a successful exploration of oxide NFs with tunnel pores via a simple single-needle microemulsion electrospinning method, which should pave the way for similar nanomaterials engineering with other chemistries for improved LSB performance.

Photothermal conversion of Ti2O3 film for tuning terahertz waves

Dynamic tuning of terahertz (THz) wave is vital for the development of next generation THz devices. Utilization of solar energy for tuning THz waves is a promising, eco-friendly, and sustainable way to expand THz application scenarios. Ti₂O₃ with an ultranarrow bandgap of 0.1eV exhibits intriguing thermal-induced metal-insulator transition (MIT), and possesses excellent photothermal conversion efficiency. Herein, Ti₂O₃ film was fabricated by a two-step magnetron sputtering method, and exhibited an excellent photothermal conversion efficiency of 90.45% and demonstrated temperature-dependent THz transmission characteristics with a wideband at 0.1-1 THz. We supposed to combine photothermal conversion characteristics with temperature-dependent THz transmission properties of Ti₂O₃ film, which could introduce solar light as the energy source for tuning THz waves. Our work will provide new sight for investigating MIT characteristics of Ti₂O₃ at THz regime and exhibit huge potential in the application of tuning terahertz waves in outdoor scenarios in the future.

Engineering a TiNb2O7-Based Electrocatalyst on a Flexible Self-Supporting Sulfur Cathode for Promoting Li-S Battery Performance

Lithium-sulfur (Li-S) batteries are considered a prospective energy storage system because of their high theoretical specific capacity and high energy density, whereas Li-S batteries still face many serious challenges on the road to commercialization, including the shuttle effect of lithium polysulfides (LiPSs), their insulating nature, the volume change of the active materials during the charge-discharge process, and the tardy sulfur redox kinetics. In this work, double transition metal oxide TiNb₂O₇ (TNO) nanometer particles are tactfully deposited on the surface of an activated carbon cloth (ACC), activating the surface through a hydrothermal reaction and high-temperature calcination and finally forming the flexible self-supporting architecture as an effective catalyst for sulfur conversion reaction. It has been found that ACC@TNO possesses many catalytic activity sites, which can inhibit the shuttle effect of LiPSs and increase the Coulombic efficiency by boosting the redox reaction kinetics of LiPS transformation reaction. As a consequence, the ACC@TNO/S cathode exhibits an impressive electrochemical performance, including a high initial discharge capacity of 885 mAh g^-1 at a high rate of 1 C, a high discharge specific capacity of 825 mAh g^-1 after 200 cycles with a prominent capacity retention rate of 93%, and a small decay rate of 0.034% per cycle. Although TNO is extensively used in the fields of lithium ion batteries and other rechargeable batteries, it is first introduced as sulfur host materials to boost the redox reaction kinetics of the LiPS transformation reaction and increase the electrochemical performance of Li-S batteries. Therefore, studies of the synergistic effect on the chemical absorption and catalytic conversion effect of TNO for LiPSs of Li-S batteries provide a good strategy for boosting further the comprehensive electrochemical performances of Li-S batteries.

Plasma-etched Ti2O3 with oxygen vacancies for enhanced NH3 electrosynthesis and Zn–N2 batteries

Plasma-etched OV-Ti2O3 behaves as an active and stable catalyst for electrochemical N2 reduction to yield NH3, capable of attaining a large NH3 yield of 37.24 μg h−1 mgcat.−1 and high faradaic efficiency of 19.29%.

Ti2O3 Nanoparticles with Ti3+ Sites toward Efficient NH3 Electrosynthesis under Ambient Conditions

Electrocatalytic nitrogen reduction reaction (NRR) enabled by introducing Ti³⁺ defect sites into TiO₂ through a doping strategy has recently attracted widespread attention. However, the amount of Ti³⁺ ions is limited due to the low concentration of dopants. Herein, we propose Ti₂O₃ nanoparticles as a pure Ti³⁺ system that performs efficiently toward NH₃ electrosynthesis under ambient conditions. This work has suggested that Ti³⁺ ions, as the main catalytically active sites, significantly increase the NRR activity. In an acidic electrolyte, Ti₂O₃ achieves extraordinary performance with a high NH₃ yield and a Faradaic efficiency of 26.01 μg h^-1 mg^-1 cat. and 9.16%, respectively, which are superior to most titanium-based NRR catalysts recently reported. Significantly, it also demonstrates a stable NH₃ yield in five consecutive cycles. Theoretical calculations uncovered that the enhanced electrocatalytic activity of Ti₂O₃ originated from Ti³⁺ active sites and significantly lowered the overpotential of the potential-determining step.

Achieving Cycling Durability of Lithium-Sulfur Batteries via Capturing Polysulfides through a Three-Dimensional Interconnected Carbon Network Anchored with Ultrafine FeS Nanoparticles

Shuttle effect has always been a critical obstacle to the application of lithium-sulfur (Li-S) batteries for leading to unstable cycle performance and a short lifespan. To solve this problem, a particular strategy is put up to relieve shuttle effect by capturing soluble polysulfides through a three-dimensional interconnected carbon network. Due to the uniformly anchored ultrafine FeS nanoparticles on a 3D interconnected carbon network, the material could lock soluble polysulfides on the cathode side and promote electrochemical conversion reactions among sulfur species. By optimizing the active site exposure of FeS and designing a hierarchical porous and multichannel structure to ensure rapid migration of ions and electrons at the same time, the interlayer can effectively suppress the shuttle effect and enhance sulfur utilization. Thus, the Li-S battery presents excellent cycling stability and rate capability, namely, a reversible specific capacity of 560 mAh g^-1 at 2.0 C over 500 cycles with a decay rate of 0.012% per cycle and a specific capacity of 597 mAh g^-1 at a 5.0 C current rate. This study offers a promising strategy for designing the structure of an interlayer to achieve long-cycle stable Li-S batteries.

Substoichiometric titanium oxide Ti2O3 exhibits greater efficiency in enhancing hydrolysis of 1,1,2,2-tetrachloroethane than TiO2 nanomaterials

Oxygen-deficient substoichiometric titanium oxides, or "titanium suboxides," are produced incidentally from coal combustion and are environmentally abundant. Additionally, titanium suboxide nanomaterials are promising new materials with likely future environmental release. How these materials may affect contaminant fate differently than stoichiometric TiO₂ (nano)materials is largely unknown. Here, we show that Ti₂O₃ (selected as a model titanium suboxide) exhibits significantly greater efficiency in enhancing the hydrolysis of 1,1,2,2-tetrachloroethane (TeCA), a common groundwater contaminant, than the stoichiometric anatase and rutile TiO₂. At environmentally relevant pH (6.5-7.5), the surface area-normalized pseudo-first-order hydrolysis rate constant in the presence of Ti₂O₃ is approximately an order of magnitude higher than those associated with TiO₂. The superior catalytic efficiency of Ti₂O₃ can be attributed to both its higher surface hydrophobicity, which renders higher adsorption affinity for TeCA, and its higher concentration of Lewis acid sites (mainly the Ti³⁺ and the five-coordinated Ti⁴⁺). Particularly, the deprotonated hydroxyl groups attached to Ti³⁺ (a weaker Lewis acid than Ti⁴⁺) exhibit higher basicity and thus, are more effective in catalyzing the base-promoted hydrolysis reaction. The findings call for further understanding of the environmental implications of titanium suboxide (nano)materials, which may not be readily predictable based on the knowledge acquired for TiO₂.

MXene-Derived Tin O2 n- 1 Quantum Dots Distributed on Porous Carbon Nanosheets for Stable and Long-Life Li-S Batteries: Enhanced Polysulfide Mediation via Defect Engineering

The application of Li-S batteries has been hindered by the shuttling behavior and sluggish reaction kinetics of polysulfides. Here an effective polysulfide immobilizer and catalytic promoter is developed by proposing oxygen-vacancy-rich Ti_n O_{2 n -1} quantum dots (OV-T_n QDs) decorated on porous carbon nanosheets (PCN), which are modulated using Ti₃ C₂ T_x MXene as starting materials. The T_n QDs not only confine polysulfides through strong chemisorption but also promote polysulfide conversion via redox-active catalysis. The introduction of oxygen vacancies further boosts the immobilization and conversion of polysulfides by lowering the adsorption energy and shortening the bond lengths. The PCN provides a physical polysulfide confinement as well as a flexible substrate preventing OV-T_n QDs from aggregation. Moreover, the two building blocks are conductive, thereby effectively improving the electron/charge transfer. Finally, the ultrasmall size of QDs along with the porous structure endows OV-T_n QDs@PCN with large specific surface area and pore volume, affording adequate space for S loading and volume expansion. Therefore, the OV-T_n QDs@PCN/S delivers a high S loading (79.1 wt%), good rate capability (672 mA h g^-1 at 2 C), and excellent long-term cyclability (88% capacity retention over 1000 cycles at 2 C). It also exhibits good Li⁺ storage under high S-mass loading and lean electrolyte.

New Insights into the N-S Bond Formation of a Sulfurized-Polyacrylonitrile Cathode Material for Lithium-Sulfur Batteries

Sulfurized polyacrylonitrile (S-cPAN) has been recognized as a particularly promising cathode material for lithium-sulfur (Li-S) batteries due to its ultra-stable cycling performance and high degree of sulfur utilization. Though the synthetic conditions and routes for modification of S-cPAN have been extensively studied, details of the molecular structure of S-cPAN remain yet unclear. Herein, a more reasonable molecular structure consisting of pyridinic/pyrrolic nitrogen (N_PD/N_PL) is proposed, based on the analysis of combined X-ray photoelectron spectroscopy, ¹³C/¹⁵N solid-state nuclear magnetic resonance, and density functional theory data. The coexistence of vicinal N_PD/N_PL entities plays a vital role in attracting S₂ molecules and facilitating N-S bond formation apart from the generally accepted C-S bond in S-cPAN, which could explain the extraordinary electrochemical features of S-cPAN among various nitrogen-containing sulfurized polymers. This study provides new insights and a better understanding of structural details and relevant bond formation mechanisms in S-cPAN, providing a foundation for the design of new types of sulfurized cathode materials suitable for application in next-generation high-performance Li-S batteries.

Critical Advances in Ambient Air Operation of Nonaqueous Rechargeable Li-Air Batteries

Over the past few years, great attention has been given to nonaqueous lithium-air batteries owing to their ultrahigh theoretical energy density when compared with other energy storage systems. Most of the research interest, however, is dedicated to batteries operating in pure or dry oxygen atmospheres, while Li-air batteries that operate in ambient air still face big challenges. The biggest challenges are H₂ O and CO₂ that exist in ambient air, which can not only form byproducts with discharge products (Li₂ O₂ ), but also react with the electrolyte and the Li anode. To this end, recent progress in understanding the chemical and electrochemical reactions of Li-air batteries in ambient air is critical for the development and application of true Li-air batteries. Oxygen-selective membranes, multifunctional catalysts, and electrolyte alternatives for ambient air operational Li-air batteries are presented and discussed comprehensively. In addition, separator modification and Li anode protection are covered. Furthermore, the challenges and directions for the future development of Li-air batteries are presented.

Constructing Co3S4 Nanosheets Coating N-Doped Carbon Nanofibers as Freestanding Sulfur Host for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur batteries (LSBs) have shown great potential as a rival for next generation batteries, for its relatively high theoretical capacity and eco-friendly properties. Nevertheless, blocked by the shuttle effect of lithium polysulfides (LPSs, Li2S4-Li2S8) and insulation of sulfur, LSBs show rapid capacity loss and cannot achieve the practical application. Herein, a composite of carbon nanofibers coated by Co3S4 nanosheets (denoted as CNF@Co3S4) is successfully synthesized as freestanding sulfur host to optimize the interaction with sulfur species. The combination of the two materials can lead extraordinary cycling and rate performance by alleviating the shuttle of LPSs effectively. N-doped carbon nanofibers serve as long-range conductive networks and Co3S4 nanosheets can accelerate the conversion of LPSs through its electrocatalytic and chemical adsorption ability. Benefiting from the unique structure, the transporting rate of Li+ can be enhanced. Distribution of Li+ is uniform for enough exposed negative active sites. As a result, the cell with CNF@Co3S4 as sulfur host is able to stabilize at 710 mA h g-1 at 1 C after 200 cycles with average coulombic efficiency of 97.8% in a sulfur loading of 1.7 mg cm-2 and deliver 4.1 mA h cm-2 at 0.1 C even in 6.8 mg cm-2 for 100 cycles.

CoP/C Nanocubes-Modified Separator Suppressing Polysulfide Dissolution for High-Rate and Stable Lithium-Sulfur Batteries

A functioned PP was chosen as a separator to suppress the shuttling effect of soluble polysulfide in lithium-sulfur batteries (LSBs). Nanocubic cobalt phosphide/carbon (CoP/C) was modified on PP membrane through a simple vacuum filtration method. This CoP/C-modified PP separator not only efficiently captures polysulfides through strong chemical affinity but also facilitates the conversion of the soluble intermediates due to the fast transfer at the interface. In consequence, the cell with a CoP/C-modified separator exhibits a low-capacity decay of only 0.08% per cycle over 500 cycles at 1 C with an initial capacity of 938 mAh g^-1 and a superior rate performance of 594 mAh g^-1 at 4 C. Even with a high loading of 3.2 mg cm^-2, the cell still exhibits an excellent reversible capacity of 601.3 mAh g^-1 after 100 cycles at 0.5 C. This work provides a new strategy to effectively restrict the polysulfide shuttling.

Calixarene-Functionalized Porous Carbon Aerogels for Polysulfide Capture: Cathodes for High Performance Lithium-Sulfur Batteries

A cage-type composite was successfully prepared by attaching p-sulfonatocalix[4]arene to a porous activated carbon aerogel (ACA). The resulting composite showed a high specific surface area of 1620.7 m²  g^-1 and a high sulfur loading of 2.5 mg cm^-2 . The calixarene is uniformly dispersed on the carbon spheres and efficiently captures polysulfides by interaction with the sulfonate groups. Meanwhile, the cross-linked porous structure of the composite restricts the migration of polysulfides. The cathode delivers an outstanding electrochemical performance with an initial capacity of 1304.7 mAh g^-1 at 0.2 C. Furthermore, it displays excellent long-term cycling stability, maintaining 884.7 mAh g^-1 after 300 cycles at 0.5 C. Density functional theory (DFT) adsorption calculations support the strong interaction between the calixarenes and polysulfides and reveal the capture mechanism.

Carbon-Coated Yttria Hollow Spheres as Both Sulfur Immobilizer and Catalyst of Polysulfides Conversion in Lithium-Sulfur Batteries

Li-S battery has tremendous application prospect on account of the high theoretical specific capacity and large energy density, while its large-scale application is impeded by the severe shuttle effect and the slow electrochemical kinetics of polysulfides conversion. Herein, the Lewis acidic yttria hollow spheres (YHS) are rationally designed as both sulfur immobilizer and catalyst of polysulfides conversion for the advanced Li-S batteries. It can be known that the Lewis acidic yttria can effectively capture the Lewis basic polysulfides and thus mitigate the shuttle effect of Li-S battery; besides, yttria shows the enhanced catalytic effect for the kinetics of interconversion reaction from polysulfides to Li₂S. As a result, either as a sulfur host or as the separator coating, yttria plays a vital part in realizing the high specific discharge capacity and good cycle stability for Li-S battery. In particular, Li-S battery with YHS@C/S cathode and YHS/CNT-0.6- modified separator (2.1 mg cm^-2 active material loading) shows a good specific discharge capacity of 912.5 mAh g^-1 at 0.5C. Even after 200 steady cycles, the discharge specific capacity can keep as 842.3 mAh g^-1, and the capacity decay rate is only 0.038% per cycle. When active material areal loading is increased to 4.24 mg cm^-2, it still maintains a considerable areal capacity of 3.79 mAh cm^-2. In consequence, the synergy of polysulfides confinement and catalytic conversion reaction provides a meaningful exploration for achieving the high performance of Li-S batteries.