Highly stable and reversible lithium storage in SnO2 nanowires surface coated with a uniform hollow shell by atomic layer deposition

Atomic Layer Deposition-A Versatile Toolbox for Designing/Engineering Electrodes for Advanced Supercapacitors

Atomic layer deposition (ALD) has become the most widely used thin-film deposition technique in various fields due to its unique advantages, such as self-terminating growth, precise thickness control, and excellent deposition quality. In the energy storage domain, ALD has shown great potential for supercapacitors (SCs) by enabling the construction and surface engineering of novel electrode materials. This review aims to present a comprehensive outlook on the development, achievements, and design of advanced electrodes involving the application of ALD for realizing high-performance SCs to date, as organized in several sections of this paper. Specifically, this review focuses on understanding the influence of ALD parameters on the electrochemical performance and discusses the ALD of nanostructured electrochemically active electrode materials on various templates for SCs. It examines the influence of ALD parameters on electrochemical performance and highlights ALD's role in passivating electrodes and creating 3D nanoarchitectures. The relationship between synthesis procedures and SC properties is analyzed to guide future research in preparing materials for various applications. Finally, it is concluded by suggesting the directions and scope of future research and development to further leverage the unique advantages of ALD for fabricating new materials and harness the unexplored opportunities in the fabrication of advanced-generation SCs.

From metal nanowires to ultrathin crystalline ALD nanotubes: process development and mechanism revealed by in situ TEM heating experiments

The creation of hollow nanomaterials based on metal oxides has become an important research topic, as they show potential in a broad range of technical applications. However, the controlled synthesis of long and at the same time thin nanotubes is still challenging. Here we present a universal approach to create ultrathin aluminum oxide nanotubes with a length/diameter ratio of >1200 and minimum wall thickness of ≤4 nm. We use a facile process based on defined heat treatment of specific core-shell nanowires. The metal nanowires act as a template, which is thermally removed during heat treatment until an empty tube is created. The core-shell nanowires are produced by Physical Vapour Deposition (PVD) with a subsequent coating via Atomic Layer Deposition (ALD). The custom-built PVD-ALD system enables a direct sample transfer without breaking the vacuum, which allows determining the effect of a native oxide layer on the metal-ALD bonding. In combination with correlative ex situ observations, in situ Transmission Electron Microscopy (TEM) heating experiments unravel the dynamical processes going on at small scales. Based on the microscopic analysis, the energetics of the core material is analyzed, giving insights about heat induced effects as well as the phase transition from the amorphous to the crystalline state.

The design and synthesis of spinel one-dimensional multi-shelled nanostructures for Li-ion batteries

The rational design, synthesis, and massive production of one-dimensional (1D) spinel composite oxides with multi-shelled nanostructures are critical for the realization of highly efficient energy conversion and storage. However, owing to the limitations of the synthetic methods, the 1D multi-shelled nanostructures, especially for multi-element oxides and binary-metal oxides, have been rarely fabricated. Herein, we design a facile and general method to fabricate 1D spinel composite oxides with complex architectures. It is found that the concentration of the precursor polymer PAN can control the structures of the products at optimal heating rate, including hollow nanofibers, wire-in-tube nanofibers, and tube-in-tube nanofibers. This technique could be extended to various inorganic multi-element oxides and binary-metal oxides. Moreover, numerous twin boundaries (TBs) are found to form in the Co_0.5Ni_0.5Fe₂O₄ tube-in-tube nanofibers. Benefiting from both large porosity and TBs structures, the tube-in-tube hollow nanostructures are measured to possess superior electrochemical performances with high energy and stability in lithium-ion storage.

Sandwich-structured graphene hollow spheres limited Mn2SnO4/SnO2 heterostructures as anode materials for high-performance lithium-ion batteries

Sn-based metal oxides and composites have been widely investigated as candidate anodes for lithium-ion batteries. However, continuous capacity fade caused by serious volumetric expansion and crystal pulverization is often noticed during lithiation and alloying processes. In this study, we design a novel heterogeneous structural composite by constructing sandwich-structured graphene hollow spheres limited Mn₂SnO₄/SnO₂ heterostructures (Mn₂SnO₄/SnO₂@SG), of which infiltration of Mn source promotes the dissolution-redeposition of SnO₂ in hollow-spherical graphene (SnO₂@SG) and their in-situ transformation into Mn₂SnO₄; and the uniform distributed Mn₂SnO₄ and SnO₂ nanoparticles are adjacent each other to form heterostructure within the sandwiched graphene hollow spheres. By comparing with the single metal oxide SnO₂@SG material, the influence of the microstructure, chemical composition, element valence state and electrochemical properties of the heterostructured Mn₂SnO₄/SnO₂@SG is investigated. The results show that the construction of Mn₂SnO₄/SnO₂ heterostructure dramatically improves electronic/ionic transport kinetics and increases lithium storage reversibility, therefore leading to distinctly superior rate capability (823.8 mAh g^-1 at 5 C) and cycling capacity. An ultra-high discharge capacity of 1180.4 mA h g^-1 is maintained up to 100 cycles at 100 mA g^-1. The promising electrochemical performances can be attributed to the sandwiched-structure hollow graphene spherical skeleton and the formation of unique Mn₂SnO₄/SnO₂ heterostructures.

Improving Lithium-Ion Half-/Full-Cell Performance of WO3 -Protected SnO2 Core-Shell Nanoarchitectures

Anodes derived from SnO₂ offer a greater specific capacity comparative to graphitic carbon in lithium-ion batteries (LIBs); hence, it is imperative to find a simple but effective approach for the fabrication of SnO₂ . The intelligent surfacing of transition metal oxides is one of the favorite strategies to dramatically boost cycling efficiency, and currently most work is primarily aimed at coating and/or compositing with carbon-based materials. Such coating materials, however, face major challenges, including tedious processing and low capacity. This study successfully reports a new and simple WO₃ coating to produce a core-shell structure on the surface of SnO₂ . The empty space permitted natural expansion for the SnO₂ nanostructures, retaining a higher specific capacity for over 100 cycles that did not appear in the pristine SnO₂ without WO₃ shell. Using WO₃ -protected SnO₂ nanoparticles as anode, a coin half-cell battery was designed with Li-foil as counter-electrode. Furthermore, the anode was paired with commercial LiFePO₄ as cathode for a coin-type full cell and tested for lithium storage performance. The WO₃ shell proved to be an effective and strong enhancer for both current rate and specific capacity of SnO₂ nanoarchitectures; additionally, an enhancement of cyclic stability was achieved. The findings demonstrate that the WO₃ can be used for the improvement of cyclic characteristics of other metal oxide materials as a new coating material.

Atomic/molecular layer deposition for energy storage and conversion

Energy storage and conversion systems, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting, have played vital roles in the reduction of fossil fuel usage, addressing environmental issues and the development of electric vehicles. The fabrication and surface/interface engineering of electrode materials with refined structures are indispensable for achieving optimal performances for the different energy-related devices. Atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques, the gas-phase thin film deposition processes with self-limiting and saturated surface reactions, have emerged as powerful techniques for surface and interface engineering in energy-related devices due to their exceptional capability of precise thickness control, excellent uniformity and conformity, tunable composition and relatively low deposition temperature. In the past few decades, ALD and MLD have been intensively studied for energy storage and conversion applications with remarkable progress. In this review, we give a comprehensive summary of the development and achievements of ALD and MLD and their applications for energy storage and conversion, including batteries, supercapacitors, fuel cells, solar cells, and photoelectrochemical water splitting. Moreover, the fundamental understanding of the mechanisms involved in different devices will be deeply reviewed. Furthermore, the large-scale potential of ALD and MLD techniques is discussed and predicted. Finally, we will provide insightful perspectives on future directions for new material design by ALD and MLD and untapped opportunities in energy storage and conversion.

ZnO/SiO2 core/shell nanowires for capturing CpG rich single-stranded DNAs

Atomic layer deposition (ALD) is capable of providing an ultrathin layer on high-aspect ratio structures with good conformality and tunable film properties. In this research, we modified the surface of ZnO nanowires through ALD for the fabrication of a ZnO/SiO2 (core/shell) nanowire microfluidic device which we utilized for the capture of CpG-rich single-stranded DNAs (ssDNA). Structural changes of the nanowires while varying the number of ALD cycles were evaluated by statistical analysis and their relationship with the capture efficiency was investigated. We hypothesized that finding the optimum number of ALD cycles would be crucial to ensure adequate coating for successful tuning to the desired surface properties, besides promoting a sufficient trapping region with optimal spacing size for capturing the ssDNAs as the biomolecules traverse through the dispersed nanowires. Using the optimal condition, we achieved high capture efficiency of ssDNAs (86.7%) which showed good potential to be further extended for the analysis of CpG sites in cancer-related genes. This finding is beneficial to the future design of core/shell nanowires for capturing ssDNAs in biomedical applications.

Design of double-shell TiO2@SnO2 nanotubes via atomic layer deposition for improved lithium storage

TiO₂@Void@SnO₂ nanotubes synthesized by atomic layer deposition (ALD) display high capacity for lithium storage.

Tin dioxide-based nanomaterials as anodes for lithium-ion batteries

The development of new electrode materials for lithium-ion batteries (LIBs) has attracted significant attention because commercial anode materials in LIBs, like graphite, may not be able to meet the increasing energy demand of new electronic devices. Tin dioxide (SnO2) is considered as a promising alternative to graphite due to its high specific capacity. However, the large volume changes of SnO2 during the lithiation/delithiation process lead to capacity fading and poor cycling performance. In this review, we have summarized the synthesis of SnO2-based nanomaterials with various structures and chemical compositions, and their electrochemical performance as LIB anodes. This review addresses pure SnO2 nanomaterials, the composites of SnO2 and carbonaceous materials, the composites of SnO2 and transition metal oxides, and other hybrid SnO2-based materials. By providing a discussion on the synthesis methods and electrochemistry of some representative SnO2-based nanomaterials, we aim to demonstrate that electrochemical properties can be significantly improved by modifying chemical composition and morphology. By analyzing and summarizing the recent progress in SnO2 anode materials, we hope to show that there is still a long way to go for SnO2 to become a commercial LIB electrode and more research has to be focused on how to enhance the cycling stability.

Dual Immobilization of SnOx Nanoparticles by N-Doped Carbon and TiO2 for High-Performance Lithium-Ion Battery Anodes

The grain aggregation engendered kinetics failure is regarded as the main reason for the electrochemical decay of nanosized anode materials. Herein, we proposed a dual immobilization strategy to suppress the migration and aggregation of SnO_x nanoparticles and corresponding lithiation products through constructing SnO_x/TiO₂@PC composites. The N-doped carbon could anchor the tin oxide particles and inhibit their aggregation during the preparation process, leading to a uniform distribution of ultrafine SnO_x nanoparticles in the matrix. Meanwhile, the incorporated TiO₂ component works as parclose to suppress the migration and coarsening of SnO_x and corresponding lithiation products. In addition, the N-doped carbon and TiO₂/Li_xTiO₂ can significantly improve the electrical and ionic conductivities of the composites, enabling a good diffusion and charge-transfer dynamics. Owing to the dual immobilization from the "synergistic effect" of N-doped carbon and the "parclose effect" of TiO₂, the conversion reaction of SnO_x remains fully reversible throughout the cycling. Thereby, the composites exhibit excellent cycling performance in half cells and can be fully utilized in full cells. This work may provide an inspiration for the rational design of tin-based anodes for high-performance lithium-ion batteries.

Al2 O3 -Assisted Confinement Synthesis of Oxide/Carbon Hollow Composite Nanofibers and Application in Metal-Ion Capacitors

Hollow micro-/nanostructures are widely explored for energy applications due to their unique structural advantages. The synthesis of hollow structures generally involves a "top-down" casting process based on hard or soft templates. Herein, a new and generic confinement strategy is developed to fabricate composite hollow fibers. A thin and homogeneous atomic-layer-deposition (ALD) Al₂ O₃ layer is employed to confine the pyrolysis of precursor fibers, which transform into metal (or metal oxide)-carbon composite hollow fibers after removal of Al₂ O₃ . Because of the uniform coating by ALD, the resultant composite hollow fibers exhibit a hollow interior from heads to ends even if they are millimeter long. V, Fe, Co, and Ni-based hollow nanofibers, demonstrating the versatility of this synthesis method, are successfully synthesized. Because of the carbon constituent, these composite fibers are particularly useful for energy applications. Herein, the as-obtained hollow V₂ O₃ -C fiber membrane is employed as a freestanding and flexible electrode for lithium-ion capacitor. The device shows an impressive energy density and a high power density.

Recent progress on hollow array architectures and their applications in electrochemical energy storage

The structural design of electrode materials is one of the most important factors that determines the electrochemical performance of energy storage devices. In recent years, hollow micro-/nanoarray structures have been widely explored for energy applications due to their unique structural advantages. Their complex hollow interior and shell arrays enable fast ion diffusion/transport, provide abundant active sites and accommodate volume changes. Moreover, the direct contact of hollow arrays with substrates enhances the mechanical stability during long-term cycling. To date, huge progress has been achieved in the rational design of various hollow array architectures. However, a review on this topic has been rarely reported. Herein, the multifunctional merits and typical synthetic strategies for hollow array structures are analyzed in detail. Furthermore, their applications in electrochemical energy storage (such as supercapacitors and batteries) are summarized. The development and challenges of hollow arrays in terms of substrates, technique improvement and material innovation are discussed. Finally, their applications for energy storage and conversion are prospected.

Effect of Oxygen Source on the Various Properties of SnO2 Thin Films Deposited by Plasma-Enhanced Atomic Layer Deposition

Herein, we performed a comparative study of plasma-enhanced atomic layer deposition (PEALD) of SnO2 films using Sn(dmamp)2 as the Sn source and either H2O plasma or O2 plasma as the oxygen source in a wide temperature range of 100–300 °C. Since the type of oxygen source employed in PEALD determines the growth behavior and resultant film properties, we investigated the growth feature of both SnO2 PEALD processes and the various chemical, structural, morphological, optical, and electrical properties of SnO2 films, depending on the oxygen source. SnO2 films from Sn(dmamp)2/H2O plasma (SH-SnO2) and Sn(dmamp)2/O2 plasma (SO-SnO2) showed self-limiting atomic layer deposition (ALD) growth behavior with growth rates of ~0.21 and 0.07–0.13 nm/cycle, respectively. SO-SnO2 films showed relatively larger grain structures than SH-SnO2 films at all temperatures. Interestingly, SH-SnO2 films grown at high temperatures of 250 and 300 °C presented porous rod-shaped surface morphology. SO-SnO2 films showed good electrical properties, such as high mobility up to 27 cm2 V−1·s−1 and high carrier concentration of ~1019 cm−3, whereas SH-SnO2 films exhibited poor Hall mobility of 0.3–1.4 cm2 V−1·s−1 and moderate carrier concentration of 1 × 1017–30 × 1017 cm−3. This may be attributed to the significant grain boundary and hydrogen impurity scattering.

SnO2 Quantum Dots: Rational Design to Achieve Highly Reversible Conversion Reaction and Stable Capacities for Lithium and Sodium Storage

SnO₂ has been considered as a promising anode material for lithium-ion batteries (LIBs) and sodium ion batteries (SIBs), but challenging as well for the low-reversible conversion reaction and coulombic efficiency. To address these issues, herein, SnO₂ quantum dots (≈5 nm) embedded in porous N-doped carbon matrix (SnO₂ /NC) are developed via a hydrothermal step combined with a self-polymerization process at room temperature. The ultrasmall size in quantum dots can greatly shorten the ion diffusion distance and lower the internal strain, improving the conversion reaction efficiency and coulombic efficiency. The rich mesopores/micropores and highly conductive N-doped carbon matrix can further enhance the overall conductivity and buffer effect of the composite. As a result, the optimized SnO₂ /NC-2 composite for LIBs exhibits a high coulombic efficiency of 72.9%, a high discharge capacity of 1255.2 mAh g^-1 at 0.1 A g^-1 after 100 cycles and a long life-span with a capacity of 753 mAh g^-1 after 1500 cycles at 1 A g^-1 . The SnO₂ /NC-2 composite also displays excellent performance for SIBs, delivering a superior discharge capacity of 212.6 mAh g^-1 at 1 A g^-1 after 3000 cycles. These excellent results can be of visible significance for the size effect of the uniform quantum dots.

Ordered SnO2 nanotube arrays of tuneable geometry as a lithium ion battery material with high longevity

Ordered arrays of straight, parallel SnO₂ nanotubes are prepared by atomic layer deposition (ALD) on inert 'anodic' aluminum oxide porous membranes serving as templates. Various thicknesses of the SnO₂ tube walls and various tube lengths are characterized in terms of morphology by scanning electron microscopy (SEM), chemical identity by X-ray photoelectron spectroscopy (XPS) and phase composition by X-ray diffraction (XRD). Their performance as negative electrode ('anode') materials for lithium-ion batteries (LIBs) is quantified at different charge and discharge rates in the absence of additives. We find distinct trends and optima for the dependence of initial capacity and long-term stability on the geometric parameters of the nanotube materials. A sample featuring SnO₂ tubes of 30 µm length and 10 nm wall thickness achieves after 780 cycles a coulombic efficiency of >99% and a specific capacity of 671 mA h g^-1. This value represents 92% of the first-cycle capacity and 86% of the theoretical value.

Flexible 3D carbon cloth as a high-performing electrode for energy storage and conversion

High-performance energy storage and conversion devices with high energy density, power density and long-term cycling life are of great importance in current consumer electronics, portable electronics and electric vehicles. Carbon materials have been widely investigated and utilized in various energy storage and conversion devices due to their excellent conductivity, mechanical and chemical stability, and low cost. Abundant excellent reviews have summarized the most recent progress and future outlooks for most of the current prime carbon materials used in energy storage and conversion devices, such as carbon nanotubes, fullerene, graphene, porous carbon and carbon fibers. However, the significance of three-dimensional (3D) commercial carbon cloth (CC), one of the key carbon materials with outstanding mechanical stability, high conductivity and flexibility, in the energy storage and conversion field, especially in wearable electronics and flexible devices, has not been systematically summarized yet. In this review article, we present a careful investigation of flexible CC in the energy storage and conversion field. We first give a general introduction to the common properties of CC and the roles it has played in energy storage and conversion systems. Then, we meticulously investigate the crucial role of CC in typical electrochemical energy storage systems, including lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries and supercapacitors. Following a description of the wide application potential of CC in electrocatalytic hydrogen evolution, oxygen evolution/reduction, full-water splitting, etc., we will give a brief introduction to the application of CC in the areas of photocatalytically and photoelectrochemically induced solar energy conversion and storage. The review will end with a brief summary of the typical superiorities that CC has in current energy conversion and storage systems, as well as providing some perspectives and outlooks on its future applications in the field. Our main interest will be focused on CC-based flexible devices due to the inherent superiority of CC and the increasing demand for flexible and wearable electronics.

Synthesis of hierarchical Sn/SnO nanosheets assembled by carbon-coated hollow nanospheres as anode materials for lithium/sodium ion batteries

Tin-based anode materials have aroused interest due to their high capacities. Nevertheless, the volume expansion problem during lithium insertion/extraction processes has severely hindered their practical application. In particular, nano–micro hierarchical structure is attractive with the integrated advantages of nano-effect and high thermal stability of the microstructure. Herein, hierarchical Sn/SnO nanosheets assembled by carbon-coated hollow nanospheres were successfully synthesized by a facile glucose-assisted hydrothermal method, in which the glucose served as both morphology-control agent and carbon source. The hierarchical Sn/SnO nanosheets exhibit excellent electrochemical performances owing to the unique configuration and carbon coating. Specifically, a reversible high capacity of 2072.2 mA h g⁻¹ was observed at 100 mA g⁻¹. Further, 964.1 mA h g⁻¹ after 100 cycles at 100 mA g⁻¹ and 820.4 mA h g⁻¹ at 1000 mA g⁻¹ after 300 cycles could be obtained. Encouragingly, the Sn/SnO also presents certain sodium ion storage properties. This facile synthetic strategy may provide new insight into fabricating high-performance Sn-based anode materials combining the advantages of both structure and carbon coating.

Hierarchical Sn/SnO nanosheets assembled by carbon-coated hollow nanospheres with promising lithium and sodium storage performances.

Bacteriophage nanofiber fabrication using near field electrospinning

M13 bacteriophage (phage) nano- and microfibers were fabricated using electrospinning. Using liquid crystalline suspension of the phage, we successfully fabricated nano- and microscale pure phage fibers. Through a near field electrospinning process, we fabricated the desired phage fiber pattern with tunable direction and spacing. In addition, we demonstrated that the resulting phage fibers could be utilized as an electrostatic-stimulus responsive actuator. The near field electrospinning would be a very useful tool to design phage-based chemical sensors, tissue regenerative materials, energy generators, metallic and semiconductor nanowires in the future.

Low-Temperature Atomic Layer Deposition of Highly Conformal Tin Nitride Thin Films for Energy Storage Devices

We present an atomic layer deposition (ALD) process for the synthesis of tin nitride (SnN_x) thin films using tetrakis(dimethylamino) tin (TDMASn, Sn(NMe₂)₄) and ammonia (NH₃) as the precursors at low deposition temperatures (70-200 °C). This newly developed ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 °C. The growth per cycle (GPC) was found to be ∼0.21 nm/cycle at 70 °C, which decreased with increasing deposition temperature. Interestingly, when the deposition temperature was between 125 and 180 °C, the GPC remained almost constant at ∼0.10 nm/cycle, which suggests an ALD temperature window, whereas upon further increasing the temperature to 200 °C, the GPC considerably decreased to ∼0.04 nm/cycle. Thermodynamic analysis via density functional theory calculations showed that the self-saturation of TDMASn would occur on an NH₂-terminated surface. Moreover, it also suggests that the condensation of a molecular precursor and the desorption of surface *NH₂ moieties would occur at lower and higher temperatures outside the ALD window, respectively. Thanks to the characteristics of ALD, this process could be used to conformally and uniformly deposit SnN_x onto an ultranarrow dual-trench Si structure (minimum width: 15 nm; aspect ratio: ∼6.3) with ∼100% step coverage. Several analysis tools such as transmission electron microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and secondary-ion mass spectrometry were used to characterize the film properties under different deposition conditions. XRD showed that a hexagonal SnN phase was obtained at a relatively low deposition temperature (100-150 °C), whereas cubic Sn₃N₄ was formed at a higher deposition temperature (175-200 °C). The stoichiometry of these thermally grown ALD-SnN_x films (Sn-to-N ratio) deposited at 150 °C was determined to be ∼1:0.93 with negligible impurities. The optoelectronic properties of the SnN_x films, such as the band gap, wavelength-dependent refractive index, extinction coefficient, carrier concentration, and mobility, were further evaluated via spectroscopic ellipsometry analysis. Finally, ALD-SnN_x-coated Ni-foam (NF) and hollow carbon nanofibers were successfully used as free-standing electrodes in electrochemical supercapacitors and in Li-ion batteries, which showed a higher charge-storage time (about eight times greater than that of the uncoated NF) and a specific capacity of ∼520 mAh/g after 100 cycles at 0.1 A/g, respectively. This enhanced performance might be due to the uniform coverage of these substrates by ALD-SnN_x, which ensures good electric contact and mechanical stability during electrochemical reactions.

Carbon framework microbelt supporting SnOx as a high performance electrode for lithium ion batteries

Facile preparation of rational SnO_x-based electrode materials with excellent electrochemical performance is highly desired for lithium ion batteries (LIBs). In this work, carbon framework microbelt supporting SnO_x nanoparticles (CFM-SnO_x) were prepared via a facile electrospinning technology and annealing treatment process. The as-synthesized CFM-SnO_x electrode exhibits high reversible capacity of 768 mAh g^-1 at 0.2 A g^-1 after 200 cycles, high rate capacity of 535 mAh g^-1 at high current density of 3.2 A g^-1. The facile synthesis and superior performance indicate that the as-synthesized CFM-SnO_x is a competitive anode material for LIBs.

Encapsulation of Fe3O4 between Copper Nanorod and Thin TiO2 Film by ALD for Lithium-Ion Capacitors

Lithium-ion capacitors (LICs) are considered to be promising power sources due to their combination of high-rate capacitors and high-capacity batteries. However, development of a high-performance LIC is still restricted by the sluggish intercalation reaction and unsatisfied specific capacities in battery-type bulk anodes. To overcome these issues, herein, we utilize two-step atomic layer deposition (ALD) to realize a uniform coating of FeO _x and TiO₂ on CuO nanorods, which results in the formation of ternary CuO@FeO _x@TiO₂ composite. After further treatment in H₂/Ar atmosphere, the as-derived Fe₃O₄ is encapsulated between conductive Cu nanorod and hollow TiO₂ shell (denoted as Cu@Fe₃O₄@TiO₂). Owing to the rational design, the binder-free Cu@Fe₃O₄@TiO₂ electrode exhibits an ultrahigh Li-ion storage capacity (1585 mA h g^-1 at 0.2 A g^-1), superior rate capability, and excellent cycle performance (no decay after 1000 cycles), which could efficiently boost the energy-storage capability of LICs. By employing an anode of Cu@Fe₃O₄@TiO₂ and a cathode of activated carbon, the as-constructed full LIC device provides high energy//powder densities (154.8 Wh kg^-1 at 200 W kg^-1; 66.2 Wh kg^-1 at 30 kW kg^-1). These superior results demonstrate that ALD-enabled architectures hold great promise for synthesizing high-capacity anodes for LICs.

Optimization of photonic nanojets generated by multilayer microcylinders with a genetic algorithm

We employ a genetic algorithm coupled with Mie theory to optimize the magnetic field intensity profile of photonic nanojets (PNJs) generated by multilayer microcylinders at visible wavelengths in free space. We first optimize five-layer microcylinders to elongate the PNJs. We show that a properly designed five-layer microcylinder structure can generate an ultra-long PNJ with a beam length ~ 107.5 times the illumination wavelength λ₀. We then optimize five-layer microcylinders to narrow the waist of PNJs. We show that a PNJ with a full-width at half maximum waist of ~ 0.22λ₀ can be obtained outside the surface of the optimized microcylinder. In addition, curved PNJs with subwavelength waist are also obtained. We finally optimize the five-layer structures for refractive index sensing based on the beam length of PNJs. The estimated minimum detectable refractive index variation when using this sensing method is ultra-small. Our results could potentially contribute to the development of a new generation of devices for optical nanoscopy and biophotonics, and greatly promote the practical applications of PNJs.

Sonochemistry-enabled uniform coupling of SnO2 nanocrystals with graphene sheets as anode materials for lithium-ion batteries

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors. In the sonochemical process, the Sn²⁺ is firstly dispersed homogeneously on the GO surface, then in situ oxidized to SnO₂ nanoparticles on both sides of the graphene nanosheets (RGO) obtained by the reduction of GO under continuous ultrasonication. Graphene not only provides a mechanical support to alleviate the volume changes of the SnO₂ anode and prevent nanoparticle agglomeration, but also serves as a conductive network to facilitate charge transfer and Li⁺ diffusion. When used as a lithium ion battery (LIB) anode, the SnO₂/graphene nanocomposite exhibits significantly improved specific capacity (1610 mA h g⁻¹ at 100 mA g⁻¹), good cycling stability (retaining 87% after 100 cycles), and competitive rate performance (273 mA h g⁻¹ at 500 mA g⁻¹) compared to those of bare SnO₂. This sonochemical method can be also applied to the synthesis of other metal-oxide/graphene composites and this work provides a large-scale preparation route for the practical application of SnO₂ in lithium ion batteries.

SnO₂/graphene nanocomposite was successfully synthesized by a facile sonochemical method from SnCl₂ and graphene oxide (GO) precursors.

Low Interface Energies Tune the Electrochemical Reversibility of Tin Oxide Composite Nanoframes as Lithium-Ion Battery Anodes

The conversion reaction of lithia can push up the capacity limit of tin oxide-based anodes. However, the poor reversibility limits the practical applications of lithia in lithium-ion batteries. The latest reports indicate that the reversibility of lithia has been appropriately promoted by compositing tin oxide with transition metals. The underlying mechanism is not revealed. To design better anodes, we studied the nanostructured metal/Li₂O interfaces through atomic-scale modeling and proposed a porous nanoframe structure of Mn/Sn binary oxides. The first-principles calculation implied that because of a low interface energy of metal/Li₂O, Mn forms smaller particles in lithia than Sn. Ultrafine Mn nanoparticles surround Sn and suppress the coarsening of Sn particles. Such a composite design and the resultant interfaces significantly enhance the reversible Li-ion storage capabilities of tin oxides. The synthesized nanoframes of manganese tin oxides exhibit an initial capacity of 1620.6 mA h g^-1 at 0.05 A g^-1. Even after 1000 cycles, the nanoframe anode could deliver a capacity of 547.3 mA h g^-1 at 2 A g^-1. In general, we demonstrated a strategy of nanostructuring interfaces with low interface energy to enhance the Li-ion storage capability of binary tin oxides and revealed the mechanism of property enhancement, which might be applied to analyze other tin oxide composites.

2D-Pnictogens: alloy-based anode battery materials with ultrahigh cycling stability

There is an increasing demand for efficient energy storage systems in our modern mobile society for a wide range of applications such as smart grids, portable electronic devices, and electric vehicles. The performance of advanced batteries in terms of energy density, power density, cyclability, and safety is mainly determined by the primary functional components, particularly by the electrode materials. Black phosphorus (BP) and the following elements in group V (pnictogens) including arsenic, antimony, and bismuth with layered structures have attracted tremendous attention to replace the graphite anode. This is due to their extremely high specific-capacities for lithium and sodium storage based on the alloying reaction mechanism; however, the same mechanism causes an irreversible volume expansion and thus low cycling stability. Since the discovery of single layer BP and its outstanding physical properties such as tunable band gap, strong in-plane anisotropy, and high carrier mobility, the battery community have intensively studied this material as well as the 2D structures of other pnictogens. In this review, first, the preparation and properties of 2D-pnictogens including crystal structure and chemical stability are briefly described. Second, the theoretical and experimental details of the intercalation and alloying mechanisms are discussed. Finally, the excellent performance of 2D-pnictogens for lithium ion and sodium ion batteries and their principal advantages compared to their parent 3D structures are presented.

Extraordinary lithium ion storage capability achieved by SnO2 nanocrystals with exposed {221} facets

Rational design of SnO2 nanomaterials with superior architectures and excellent electrochemical properties is highly desirable for lithium ion storage. Here, several SnO2 nanoparticles with different exposed crystal planes, such as {101}, {110} and {221} facets, are developed and further embedded into graphene/carbon nanotube (G/CNT) networks, achieving highly conductive carbon/SnO2 films (C/SnO2) with homogeneous dispersion of SnO2 nanoparticles. Three-dimensional (3D) G/CNT networks with highly porous structures and electronic contacts with imbedded SnO2 nanoparticles provide excellent pathways for transfer of electrons and ions and further buffer structural changes of SnO2 nanocrystals during lithium-ion insertion/extraction processes. Close contact between G/CNT matrix and embedded SnO2 nanoparticles ensures that all high-energy {221} facets of SnO2 are exploited during rapid electrochemical reactions. The high electrical conductivity of G/CNT networks can further prevent pulverization of nanostructured SnO2. As a result, C/SnO2 film with 90% content of octahedral SnO2 nanocrystals (C/SnO2-O (90%)) exhibits superior reversible specific capacity of 1008 mA h g-1 at 0.1 A g-1, excellent rate capability, low internal resistance and long-term cycling stability for 1000 cycles. These results further confirm that SnO2 nanocrystals with high-energy {221} facets can provide numerous active sites for lithium ion storage than other SnO2 nanomaterials.

Hollow SnO2 nanospheres with oxygen vacancies entrapped by a N-doped graphene network as robust anode materials for lithium-ion batteries

The practical application of tin dioxide (SnO2) in lithium-ion batteries has been greatly hindered by its large volumetric expansion and low conductivity. Thus, a rational design of the size, geometry and the pore structure of SnO2-based nanomaterials is still a dire demand. To this end, herein we report an effective approach for engineering hollow-structured SnO2 nanospheres with adequate surface oxygen vacancies simultaneously wrapped by a nitrogen-doped graphene network (SnO2-x/N-rGO) through an electrostatic adsorption-induced self-assembly together with a thermal reduction process. The close electrostatic attraction achieved a tight and uniform combination of positively charged SnO2 nanospheres with negatively charged graphene oxide (GO), which can alleviate the aggregation and volume expansion of the entrapped SnO2 nanospheres. Subsequent thermal treatment not only ensures a significant reduction of the GO sheets accompanying nitrogen-doping, but also induces the generation of oxygen vacancies on the surface of the SnO2 hollow nanospheres, together building up a long-range and bicontinuous transfer channel for rapid electron and ion transport. Because of these structural merits, the as-built SnO2-x/N-rGO composite used as the anode material exhibits excellent robust cycling stability (∼912 mA h g-1 after 500 cycles at 0.5 A g-1 and 652 mA h g-1 after 200 cycles at 1 A g-1) and superior rate capability (309 mA h g-1 at 10 A g-1). This facile fabrication strategy may pave the way for the construction of high performance SnO2-based anode materials for potential application in advanced lithium-ion batteries.

Sn-Based Nanocomposite for Li-Ion Battery Anode with High Energy Density, Rate Capability, and Reversibility

To design an easily manufactured, large energy density, highly reversible, and fast rate-capable Li-ion battery (LIB) anode, Co-Sn intermetallics (CoSn₂, CoSn, and Co₃Sn₂) were synthesized, and their potential as anode materials for LIBs was investigated. Based on their electrochemical performances, CoSn₂ was selected, and its C-modified nanocomposite (CoSn₂/C) as well as Ti- and C-modified nanocomposite (CoSn₂/ a-TiC/C) was straightforwardly prepared. Interestingly, the CoSn₂, CoSn₂/C, and CoSn₂/ a-TiC/C showed conversion/nonrecombination, conversion/partial recombination, and conversion/full recombination during Li insertion/extraction, respectively, which were thoroughly investigated using ex situ X-ray diffraction and extended X-ray absorption fine structure analyses. As a result of the interesting conversion/full recombination mechanism, the easily manufactured CoSn₂/ a-TiC/C nanocomposite for the Sn-based Li-ion battery anode showed large energy density (first reversible capacity of 1399 mAh cm^-3), high reversibility (first Coulombic efficiency of 83.2%), long cycling behavior (100% capacity retention after 180 cycles), and fast rate capability (appoximately 1110 mAh cm^-3 at 3 C rate). In addition, degradation/enhancement mechanisms for high-capacity and high-performance Li-alloy-based anode materials for next-generation LIBs were also suggested.

Sn-Doped Rutile TiO2 Hollow Nanocrystals with Enhanced Lithium-Ion Batteries Performance

Hollow structures and doping of rutile TiO₂ are generally believed to be effective ways to enhance the performance of lithium-ion batteries. Herein, uniformly distributed Sn-doped rutile TiO₂ hollow nanocrystals have been synthesized by a simple template-free hydrothermal method. A topotactic transformation mechanism of solid TiOF₂ precursor is proposed to illustrate the formation of rutile TiO₂ hollow nanocrystals. Then, the Sn-doped rutile TiO₂ hollow nanocrystals are calcined and tested as anode in the lithium-ion battery. They deliver a highly reversible specific capacity of 251.3 mA h g^-1 at 0.1 A g^-1 and retain ∼110 mA h g^-1 after 500 cycles at a high current rate 5 A g^-1 (30 C), which is much higher than most of the reported work.

Stable and Reversible Lithium Storage with High Pseudocapacitance in GaN Nanowires

In this work, gallium nitride (GaN) nanowires (NWs) were synthesized by chemical vapor deposition (CVD) process. The hybrid electrode showed the capacity up to 486 mAh g^-1 after 400 cycles at 0.1 A g^-1. Even at 10 A g^-1, the reversible capacity can stabilize at 75 mAh g^-1 (after 1000 cycles). Pseudocapacitive capacity was defined by kinetics analysis. The dynamics analysis and electrochemical reaction mechanism of GaN with Li⁺ was also analyzed by ex situ XRD, HRTEM, and XPS results. These results not only cast new light on pseudocapacitance enhanced high-rate energy storage devices by self-assembled nanoengineering but also extend the application range of traditional binary III/V semiconductors.

Sn Wears Super Skin: A New Design for Long Cycling Batteries

Searching for new anode alternatives in lieu of graphite for lithium-ion batteries that can deliver better electrochemical performance to meet the emerging energy/power demands in electric vehicles becomes particularly challenging. We report a rationally designed hybrid composite as anode in LIB that exhibits a greatly improved gravimetric capacity of 727 mAh/g with a Coulombic efficiency of >99.8% after 3000 cycles at 1.0 C. A capacity of 662 mAh/g at a high rate of 5.0 C was obtained after impressively long 10 000 cycles. From the 50th to 10 000th cycle under 5.0 C, the capacity retention is >97% with a negligible decay of <0.00026% per cycle. The excellence in electrochemistry is attributed to the efficient stress relax, accommodable space, lack of agglomeration, and solid-electrolyte interphase consuming Li⁺ of a delicate composite configuration that is composed of a Sn kernel wearing adjustable TiO₂ "skin".

Growth of SnO2 Nanoflowers on N-doped Carbon Nanofibers as Anode for Li- and Na-ion Batteries

It is urgent to solve the problems of the dramatic volume expansion and pulverization of SnO₂ anodes during cycling process in battery systems. To address this issue, we design a hybrid structure of N-doped carbon fibers@SnO₂ nanoflowers (NC@SnO₂) to overcome it in this work. The hybrid NC@SnO₂ is synthesized through the hydrothermal growth of SnO₂ nanoflowers on the surface of N-doped carbon fibers obtained by electrospinning. The NC is introduced not only to provide a support framework in guiding the growth of the SnO₂ nanoflowers and prevent the flower-like structures from agglomeration, but also serve as a conductive network to accelerate electronic transmission along one-dimensional structure effectively. When the hybrid NC@SnO₂ was served as anode, it exhibits a high discharge capacity of 750 mAh g^-1 at 1 A g^-1 after 100 cycles in Li-ion battery and 270 mAh g^-1 at 100 mA g^-1 for 100 cycles in Na-ion battery, respectively.

A novel route to prepare N-graphene/SnO2 composite as a high-performance anode for lithium batteries

In this study, we report a simple method to prepare nitrogen-doped graphene, with which a nitrogen-doped graphene/SnO₂ composite was successfully fabricated and employed as a lithium battery anode.

1D Nanomaterials: Design, Synthesis, and Applications in Sodium-Ion Batteries

Sodium-ion batteries (SIBs) have received extensive attention as ideal candidates for large-scale energy storage systems (ESSs) owing to the rich resources and low cost of sodium (Na). However, the larger size of Na⁺ and the less negative redox potential of Na⁺ /Na result in low energy densities, short cycling life, and the sluggish kinetics of SIBs. Therefore, it is necessary to develop appropriate Na storage electrode materials with the capability to host larger Na⁺ and fast ion diffusion kinetics. 1D materials such as nanofibers, nanotubes, nanorods, and nanowires, are generally considered to be high-capacity and stable electrode materials, due to their uniform structure, orientated electronic and ionic transport, and strong tolerance to stress change. Here, the synthesis of 1D nanomaterials and their applications in SIBs are reviewed. In addition, the prospects of 1D nanomaterials on energy conversion and storage as well as the development and application orientation of SIBs are presented.

One-Pot Synthesis of Co-Based Coordination Polymer Nanowire for Li-Ion Batteries with Great Capacity and Stable Cycling Stability

Nanowire coordination polymer cobalt-terephthalonitrile (Co-BDCN) was successfully synthesized using a simple solvothermal method and applied as anode material for lithium-ion batteries (LIBs). A reversible capacity of 1132 mAh g^-1 was retained after 100 cycles at a rate of 100 mA g^-1, which should be one of the best LIBs performances among metal organic frameworks and coordination polymers-based anode materials at such a rate. On the basis of the comprehensive structural and morphology characterizations including fourier transform infrared spectroscopy, ¹H NMR, ¹³C NMR, and scanning electron microscopy, we demonstrated that the great electrochemical performance of the as-synthesized Co-BDCN coordination polymer can be attributed to the synergistic effect of metal centers and organic ligands, as well as the stability of the nanowire morphology during cycling.