Mesoporous Carbon Interlayers with Tailored Pore Volume as Polysulfide Reservoir for High-Energy Lithium–Sulfur Batteries

The lithium–sulfur (Li–S) battery is one of the most promising candidates for the next generation of rechargeable batteries owing to its high theoretical energy density, which is 4- to 5-fold greater than those of state-of-the-art Li–ion batteries. However, its commercial applications have been hampered due to the insulating nature of sulfur and the poor cycling stability caused by the polysulfide shuttle phenomenon. In this work, we show that Li–S batteries with a mesoporous carbon interlayer placed between the separator and the sulfur cathode not only reduces the internal resistance of the cells but also that its intrinsic mesoporosity provides a physical place for trapping soluble polysulfides as well as to alleviate the negative impact of the large volume change of sulfur. This improvement of the active material reutilization allows one to obtain a stable capacity of 1015 mAh g^–1 at 0.2 C after 200 cycles despite the use of a conventional sulfur–carbon black mixture as cathode. Furthermore, we observe an excellent capacity retention (∼0.1% loss per cycle, after the second cycle), thus making one step closer toward feasible Li–S battery technology for applications in electric vehicles and grid-scale stationary energy storage systems

Tailoring Hollow Silicon–Carbon Nanocomposites As High-Performance Anodes in Secondary Lithium-Based Batteries through Economical Chemistry

A sustainable synthesis procedure of a rational-designed silicon–carbon electrode for a high-performance rechargeable Li-based battery has been developed. It was realized by an economical approach using low-cost trichlorosilane as feedstock and without special equipment. The synthesis strategy includes polycondensation of trichlorosilane in the presence of a surfactant to selectively form spheric silicon@silica particles via a hydrogen silsesquioxane (HSQ) intermediate. After subsequent carbonization of a sucrose shell and etching the composite, we obtained an anode material based on silicon nanoparticles with 2–5-nm average diameter inside a porous carbon scaffold. The active material exhibits a high rate capability of 2000 mAh/g at a current rate of 0.5 A/g with exceptional cycle stability. After almost 1000 times of deep discharge galvanostatic cycling at 2.5 A/g current rate the capacity is still 60% of the initial 1200 mAh/g. The excellent electrochemical performance is attributed to an interaction of a stabilized solid electrolyte interface on extreme small silicon particles and a well-designed porous carbon cage which serves as efficient charge conductor

Surface and Electrochemical Studies on Silicon Diphosphide as Easy-to-Handle Anode Material for Lithium-Based Batteriesthe Phosphorus Path

The electrochemical characteristics of silicon diphosphide (SiP₂) as a new anode material for future lithium-ion batteries (LIBs) are evaluated. The high theoretical capacity of about 3900 mA h g^–1 (fully lithiated state: Li₁₅Si₄ + Li₃P) renders silicon diphosphide as a highly promising candidate to replace graphite (372 mA h g^–1) as the standard anode to significantly increase the specific energy density of LIBs. The proposed mechanism of SiP₂ is divided into a conversion reaction of phosphorus species, followed by an alloying reaction forming lithium silicide phases. In this study, we focus on the conversion mechanism during cycling and report on the phase transitions of SiP₂ during lithiation and delithiation. By using ex situ analysis techniques such as X-ray powder diffraction, formed reaction products are identified. Magic angle spinning nuclear magnetic resonance spectroscopy is applied for the characterization of long-range ordered compounds, whereas X-ray photoelectron spectroscopy gives information of the surface-layer species at the interface of active material and electrolyte. Our SiP₂ anode material shows a high initial capacity of about 2700 mA h g^–1, whereas a fast capacity fading during the first few cycles occurs which is not necessarily expected. On the basis of our results, we conclude that besides other degradation effects, such as electrolyte decomposition and electrical contact loss, the rapid capacity fading originates from the formation of a low ion-conductive layer of LiP. This insulating layer hinders lithium-ion diffusion during lithiation and thereby mainly contributes to fast capacity fading

Sandwich-Stacked SnO₂/Cu Hybrid Nanosheets as Multichannel Anodes for Lithium Ion Batteries

We have introduced a facile strategy to fabricate sandwich-stacked SnO₂/Cu hybrid nanosheets as multichannel anodes for lithium-ion batteries applying rolled-up nanotechnology with the use of carbon black as intersheet spacer. By employing a direct self-rolling and compressing approach, a much higher effective volume efficiency is achieved as compared to rolled-up hollow tubes. Benefiting from the nanogaps formed between each neighboring sheet, electron transport and ion diffusion are facilitated and SnO₂/Cu nanosheet overlapping is prevented. As a result, the sandwich-stacked SnO₂/Cu hybrid nanosheets exhibit a high reversible capacity of 764 mAh g^–1 at 100 mA g^–1 and a stable cycling performance of ∼75% capacity retention at 200 mA g^–1 after 150 cycles, as well as a superior rate capability of ∼470 mAh g^–1 at 1 A g^–1. This synthesis approach presents a promising route to design multichannel anodes for high performance Li-ion batteries

Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum Activation

Incorporating aluminum is the most widely applied and industrially relevant method to functionalize amorphous silica. However, established protocols yield predominately poorly distributed and inaccessible Al species, and as a result only ∼10–15% of the present aluminum gives rise to the acid sites, hampering the overall catalytic potential. Herein, the influence of alkaline activations with aqueous NaOH and NH₄OH on the porosity, acidity, and catalytic properties of Al-MCM-41 is studied. By performing room temperature activations in 0.01–0.1 M NaOH or 0.5 M NH₄OH, the Ostwald ripening of silica in alkaline media is exploited, which results in high mass retention yields (100–74%) and a controlled transformation of the 3.6 nm mesopores of the parent material to a broad pore range from 3 to ∼12 nm. Electron microscopy indicates the presence of additional interconnected intraparticle porosity, whereas no significant change in the shape and size of the original particles is observed. Elemental analysis reveals that the optimal alkaline activation with 0.05 M NaOH leads to a decrease in the Si/Al ratio at the surface, despite an increase in the bulk Si/Al ratio. ²⁷Al magic angle spinning nuclear magnetic resonance spectroscopy demonstrates a large conversion of octahedral Al into tetrahedral Al, doubling the purely tetrahedral fraction from 30 to 60%. Pyridine-probed Fourier transformed infrared spectroscopy shows a doubling of the Brønsted and Lewis acidity after activation. The compositional and spectroscopic results are ratified by monitoring the relative accessibility of the acid sites, i.e., effective acidity (mol acid sites per mol Al). The alkaline activation enhances the effective acidity by increasing access to the Al sites trapped inside the pore wall and by reincorporation of the octahedral Al as accessible tetrahedral sites. As a result, an unprecedented effective acidity is obtained after the Al incorporation, which is substantiated using a novel accessibility concept. The catalytic potential of the activation protocol is demonstrated by quadrupling the catalytic activity for the acid-catalyzed alkylation of toluene with benzyl alcohol, an over-50% activity gain, a slightly enhanced selectivity, and a strongly reduced coking in the acid-catalyzed coupling of furfural with sylvan

Titania-Silica Catalysts for Lactide Production from Renewable Alkyl Lactates: Structure–Activity Relations

Different Ti-Si catalysts, viz. TiO₂ supported on amorphous SiO₂ or Si-MCM-41, TiO₂-SiO₂ xerogels, and Ti zeolites (TS-1 and Ti-beta), were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88–92% at conversions below 50%. From DR UV–vis spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO₄ sites is higher than those of polymerized TiO₅ or the octahedral TiO₆ counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO₄ sites may be attributed to an increased number of vacant sites (e.g., two for TiO₄, zero for TiO₆). Lactide productivity thus highly benefits from an increased dispersion of Ti sites on the catalyst surface and could be increased by a factor of 2.5 (up to 10 g_LD g_cat^–1 h^–1) when TiO₂ is dispersed on a Si-MCM-41 support, with higher surface areas in comparison to amorphous SiO₂ gels

Synergistically Enhanced Polysulfide Chemisorption Using a Flexible Hybrid Separator with N and S Dual-Doped Mesoporous Carbon Coating for Advanced Lithium–Sulfur Batteries

Because of the outstanding high theoretical specific energy density of 2600 Wh kg^–1, the lithium–sulfur (Li–S) battery is regarded as a promising candidate for post lithium-ion battery systems eligible to meet the forthcoming market requirements. However, its commercialization on large scale is thwarted by fast capacity fading caused by the Achilles’ heel of Li–S systems: the polysulfide shuttle. Here, we merge the physical features of carbon-coated separators and the unique chemical properties of N and S codoped mesoporous carbon to create a functional hybrid separator with superior polysulfide affinity and electrochemical benefits. DFT calculations revealed that carbon materials with N and S codoping possess a strong binding energy to high-order polysulfide species, which is essential to keep the active material in the cathode side. As a result of the synergistic effect of N, S dual-doping, an advanced Li–S cell with high specific capacity and ultralow capacity degradation of 0.041% per cycle is achieved. Pushing our simple-designed and scalable cathode to a highly increased sulfur loading of 5.4 mg cm^–2, the Li–S cell with the functional hybrid separator can deliver a remarkable areal capacity of 5.9 mAh cm^–2, which is highly favorable for practical applications

Tunable Pseudocapacitance in 3D TiO_2−δ Nanomembranes Enabling Superior Lithium Storage Performance

Nanostructured TiO₂ of different polymorphs, mostly prepared by hydro/solvothermal methods, have been extensively studied for more than a decade as anode materials in lithium ion batteries. Enormous efforts have been devoted to improving the electrical conductivity and lithium ion diffusivity in chemically synthesized TiO₂ nanostructures. In this work we demonstrate that 3D Ti³⁺-self-doped TiO₂ (TiO_2−δ) nanomembranes, which are prepared by physical vapor deposition combined with strain-released rolled-up technology, have a great potential to address several of the long-standing challenges associated with TiO₂ anodes. The intrinsic electrical conductivity of the TiO₂ layer can be significantly improved by the <i>in situ</i> generated Ti³⁺, and the amorphous, thin TiO₂ nanomembrane provides a shortened Li⁺ diffusion pathway. The fabricated material shows a favorable electrochemical reaction mechanism for lithium storage. Further, post-treatments are employed to adjust the Ti³⁺ concentration and crystallinity degree in TiO₂ nanomembranes, providing an opportunity to investigate the important influences of Ti³⁺ self-doping and amorphous structures on the electrochemical processes. With these experiments, the pseudocapacitance contributions in TiO₂ nanomembranes with different crystallinity degree are quantified and verified by an in-depth kinetics analysis. Additionally, an ultrathin metallic Ti layer can be included, which further improves the lithium storage properties of the TiO₂, giving rise to the state-of-the-art capacity (200 mAh g^–1 at 1 C), excellent rate capability (up to 50 C), and ultralong lifetime (for 5000 cycles at 10 C, with an extraordinary retention of 100%) of TiO₂ anodes

Vertical Graphene Growth from Amorphous Carbon Films Using Oxidizing Gases

Amorphous carbon thin films are technologically important materials that range in use from the semiconductor industry to corrosion-resistant films. Their conversion to crystalline graphene layers has long been pursued; however, typically this requires excessively high temperatures. Thus, crystallization routes which require reduced temperatures are important. Moreover, the ability to crystallize amorphous carbon at reduced temperatures without a catalyst could pave the way for practical graphene synthesis for device fabrication without the need for transfer or post-transfer gate deposition. To this end we demonstrate a practical and facile method to crystallize deposited amorphous carbon films to high quality graphene layers at reduced annealing temperatures by introducing oxidizing gases during the process. The reactive gases react with regions of higher strain (energy) in the system and accelerate the graphitization process by minimizing criss-cross-linkages and accelerating C–C bond rearrangement at defects. In other words, the movement of crystallite boundaries is accelerated along the carbon hexagon planes by removing obstacles for crystallite coalescence

Insights into the Early Growth of Homogeneous Single-Layer Graphene over Ni–Mo Binary Substrates

The employment of Ni–Mo films has recently been shown to yield strictly homogeneous single-layer graphene. In this study, we systematically investigate the different stages of nucleation and growth of graphene over Ni–Mo layers. The studies reveal that the Ni film breaks up and diffuses into the underlying Mo foil, forming a Ni–Mo intermetallic. Nucleation only occurs from Ni sites, and thus, the nucleation density can be controlled by the Ni film thickness. Both nucleation and growth of the graphene are shown to be susceptible to very efficient self-termination processes to the formation of molybdenum carbide, and this guarantees the formation of large area graphene that consists <i>entirely</i> of monolayer graphene