Germanium Nanotubes Prepared by Using the Kirkendall Effect as Anodes for High-Rate Lithium Batteries

ZnO/Ge core-shell nanowires and Ge nanotubes fabricated by chemical vapor deposition and wet etching

One-dimensional germanium (Ge)-related nanostructures including core-shell nanowires and nanotubes with high specific surface area show enhanced performance in energy storage and electronic devices, and their structural control is important for further improving their performance and stability. In this work, we fabricated vertically formed ZnO/Ge core-shell nanowires with different shell thicknesses. The dependence of morphology, crystallinity, and internal stress of the nanowires on the shell growth time and temperature was investigated. By applying the wet-etching method to the ZnO/Ge core-shell heterojunction nanowires, we demonstrated the Ge nanotube fabrication and stress relaxation in Ge after ZnO core removal.

A Ge/Carbon Atomic-Scale Hybrid Anode Material: A Micro-Nano Gradient Porous Structure with High Cycling Stability

The continuous growth of the solid-electrolyte interface (SEI) and material crushing are the fundamental issues that hinder the application of Ge anodes in lithium-ion batteries. Solving Ge deformation crushing during discharge/charge cycles is challenging using conventional carbon coating modification methods. Due to the chemical stability and high melting point of carbon (3500 °C), Ge/carbon hybridization at the atomic level is challenging. By selecting a suitable carbon source and introducing an active medium, we have achieved the Ge/carbon doping at the atom-level, and this Ge/carbon anode shows excellent electrochemical performance. The reversible capacity is maintained at 1127 mAh g^-1 after 1000 cycles (2 A g^-1 (2-71 cycles), 4 A g^-1 (72-1000 cycles)) with a retention of 84 % compared to the second cycle. The thickness of the SEI is only 17.4 nm after 1000 cycles. The excellent electrochemical performance and stable SEI fully reflect the application potential of this material.

Recent Developments of Nanomaterials and Nanostructures for High-Rate Lithium Ion Batteries

Lithium ion batteries have been considered as a promising energy-storage solution, the performance of which depends on the electrochemical properties of each component, including cathode, anode, electrolyte and separator. Currently, fast charging is becoming an attractive research field due to the widespread application of batteries in electric vehicles, which are designated to replace conventional diesel automobiles in the future. In these batteries, rate capability, which is closely linked to the topology and morphology of electrode materials, is one of the determining parameters of interest. It has been revealed that nanotechnology is an exceptional tool in designing and preparing cathodes and anodes with outstanding electrochemical kinetics due to the well-known nanosizing effect. Nevertheless, the negative effects of applying nanomaterials in electrodes sometimes outweigh the benefits. To better understand the exact function of nanostructures in solid-state electrodes, herein, a comprehensive review is provided beginning with the fundamental theory of lithium ion transport in solids, which is then followed by a detailed analysis of several major factors affecting the migration of lithium ions in solid-state electrodes. The latest developments in characterisation techniques, based on either electrochemical or radiology methodologies, are covered as well. In addition, state-of-the-art research findings are provided to illustrate the effect of nanomaterials and nanostructures in promoting the rate performance of lithium ion batteries. Finally, several challenges and shortcomings of applying nanotechnology in fabricating high-rate lithium ion batteries are summarised.

Sb nanocrystal-anchored hollow carbon microspheres for high-capacity and high-cycling performance lithium-ion batteries

There is a great need to develop sustainable and clean energy storage devices and systems of high-energy and high-capacity densities. In this work, we synthesize antimony (Sb) nanocrystal-anchored hollow carbon microspheres (Sb@HCMs) via the calcination of cultivated yeast cells and the reduction of SbCl₃ in an ethylene glycol solution on the surface of hollow carbon microspheres. The Sb@HCMs possess hollow and porous structure, and the Sb is present in the form of nanocrystals. Using the Sb@HCMs as the active-electrode material, we assemble lithium (Li)-ion half cells and full cells and investigate their electrochemical performance. The Li-ion half cells possess a charge capacity of 605 mA h g^-1 after 100 cycles at a current density of 100 mA g^-1 and a charge capacity of 469.9 mA h g^-1 at a current density up to 1600 mA g^-1, which is much higher than the theoretical capacity of 372 mA h g^-1 for commercial graphite electrode. The Li-ion full cells with Sb@HCMs//LiCoO₂ deliver a charge capacity of 300 mA h g^-1 at a current density of 0.2 A g^-1 after 50 cycles, and have potential in applications of energy storage.

Probing the Origin of Gold Dissolution and Tunneling Across Ni2P Shell Using in situ Transmission Electron Microscopy

Hybrid core-shell catalysts possess superior physiochemical properties that are closely related to their atomic structures. Here, we report novel diffusion phenomena in an Au-Ni₂P yolk-shell structure using in situ heating transmission electron microscopy (TEM) analysis. The Au yolks can dissolve completely into and diffuse across the Ni₂P shell at 500 °C, resulting in an inward volume expansion of the Ni₂P shell and shrinkage of the void. The dissolved Au is subsequently segregated, forming a new crystal on the outer layer of the shell. A detailed atomic-scale resolution imaging revealed that the outward Au diffusion and aggregation occur when the Au yolks are epitaxial to the Ni₂P shells. Theoretical analysis shows that the outward diffusion and deposition of Au atoms is primarily driven by the excess epitaxial strain energy.

Solid-to-Hollow Conversion of Silver Nanocrystals by Surface-Protected Etching

Although hollow silver nanocrystals possess unique plasmonic properties, there is a lack of robust strategies to synthesize such nanocrystals with high efficiency and controllability. To solve this problem, a new surface-protected etching strategy to convert solid Ag nanocrystals, which are widely available from conventional syntheses, into their hollow counterparts, producing a family of hollow Ag nanocrystals is reported. Hollow Ag nanospheres and nanotubes were prepared conveniently in this way. The key was the surface modification of Ag nanocrystals by a minor amount of Pt prior to a controllable etching process, which accounts for enhanced stability of the Ag surface and subsequent etching of Ag from the inner part of the nanocrystals while retaining the overall crystal morphology. These hollow Ag nanocrystals showed distinctive optical properties, as demonstrated by the enhanced optical transmittance of flexible electrodes fabricated with Ag nanotubes, compared to nanowires. These hollow Ag nanocrystals hold promise in different plasmonic and electronic applications.

Evolution of the topological properties of two-dimensional group IVA materials and device design

Two-dimensional group IVA materials (graphene, silicene, germanene, stanene, and plumbene) are promising candidates for realization of the quantum spin Hall effect and for future device applications.

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation.

Owing to their unique properties, hollow metal nanocrystals demonstrate greater catalytic promise than their solid counterparts. Here the authors produce hollow and inflated palladium nanocrystals with thin shells via a repeated Kirkendall cavitation process, and demonstrate their activity for formic acid oxidation.

Nitrogen-Plasma-Activated Hierarchical Nickel Nitride Nanocorals for Energy Applications

Developing transition metal nitrides with unique nanomorphology is important for many energy storage and conversion processes. Here, a facile and novel one-step approach of growing 3D hierarchical nickel nitride (hNi₃ N) on Ni foam via nitrogen plasma is reported. Different from most conventional chemical synthesis, the hNi₃ N is obtained in much shorter growth duration (≤15 min) without any hazardous or reactive sources and oxide precursors at a moderate reaction zone temperature of ≤450 °C. Among possible multifunctionalities of the obtained nanocoral hNi₃ N, herein the performance in reversible lithium ion storage and electrocatalytic oxygen evolution reaction (OER) is demonstrated. The as-obtained hNi₃ N delivers a considerable cycling performance and rate stability as a lithium ion battery anode, and its property can be further enhanced by coating the hNi₃ N surface with graphene quantum dots. The hNi₃ N also serves as an active OER catalyst with high activity and stability. Additionally, on the basis of controlled growth under different nitrogen plasma treatment time, the formation mechanism of the nanocoralline hNi₃ N is outlined for further extension to other materials. The results on time- and energy-efficient nitrogen-plasma-based preparation of hNi₃ N pave the way for the development of high-performance metal nitride electrodes for energy storage and conversion.

Porous One-Dimensional Nanomaterials: Design, Fabrication and Applications in Electrochemical Energy Storage

Electrochemical energy storage technology is of critical importance for portable electronics, transportation and large-scale energy storage systems. There is a growing demand for energy storage devices with high energy and high power densities, long-term stability, safety and low cost. To achieve these requirements, novel design structures and high performance electrode materials are needed. Porous 1D nanomaterials which combine the advantages of 1D nanoarchitectures and porous structures have had a significant impact in the field of electrochemical energy storage. This review presents an overview of porous 1D nanostructure research, from the synthesis by bottom-up and top-down approaches with rational and controllable structures, to several important electrochemical energy storage applications including lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, lithium-oxygen batteries and supercapacitors. Highlights of porous 1D nanostructures are described throughout the review and directions for future research in the field are discussed at the end.

ZnO decorated germanium nanoparticles as anode materials in Li-ion batteries

Germanium exhibits high charge capacity and high lithium diffusivity, both are the key requirements for electrode materials in high performance lithium ion batteries (LIBs). However, high volume expansion and segregation from the electrode during charge-discharge cycling have limited use of germanium in LIBs. Here, we demonstrate that ZnO decorated Ge nanoparticles (Ge@ZnO NPs) can overcome these limitations of Ge as an LIB anode material. We produced Ge NPs at high rates by laser pyrolysis of GeH₄, then coated them with solution phase synthesized ZnO NPs. Half-cell tests revealed dramatically enhanced cycling stability and higher rate capability of Ge@ZnO NPs compared to Ge NPs. Enhancements arise from the core-shell structure of Ge@ZnO NPs as well as production of metallic Zn from the ZnO layer. These findings not only demonstrate a new surface treatment for Ge NPs, but also provide a new opportunity for development of high-rate LIBs.

Highly Reversible and Superior Li-Storage Characteristics of Layered GeS2 and Its Amorphous Composites

A layered GeS₂ material was assessed as an electrode material in the fabrication of superior rechargeable Li-ion batteries. The electrochemical Li insertion/extraction behavior of the GeS₂ electrode was investigated from extended X-ray absorption measurements as well as by cyclic voltammetry and differential capacity plots to better understand its Li insertion/extraction behavior. Using the Li insertion/extraction reaction mechanism of the GeS₂ electrode, an interesting amorphous GeS₂-based composite was developed and tested for use as a high-performance electrode. Interestingly, the amorphous GeS₂-based composite electrode exhibited highly reversible discharging and charging reactions, which were attributed to a conversion/recombination reaction. The amorphous GeS₂-based composite electrode exhibited highly reversible and outstanding electrochemical performances, a highly reversible capacity (first charge capacity: 1298 mAh g^-1) with a high first Coulombic efficiency (83.3%), rapid rate capability (ca. 800 mAh g^-1 at a high current rate of 700 mA g^-1), and long capacity retention over 180 cycles with high capacity (1100 mAh g^-1) thanks to its interesting electrochemical reaction mechanism. Overall, this layered GeS₂ and its amorphous GeS₂/C composite are novel alternative anode materials for the potential mass production of rechargeable Li-ion batteries with excellent performance.

Rational synthesis of carbon-coated hollow Ge nanocrystals with enhanced lithium-storage properties

High-capacity anode materials based on alloy-type group IV elements always have large volume expansion during lithiation when they are used in lithium-ion batteries. Designing hollow structures is a well-established strategy to accommodate the volume change because of sufficient internal void space. Here we report a facile template-free route to prepare hollow Ge nanospheres without using any templates through a quasi-microemulsion method. Ge nanocrystals are preferably self-assembled along the interface of liquid vesicles between water and tetrahydrofuran, and well-defined hollow architectures of ∼50 nm in diameter are formed. Both the wall thickness and hollow interiors can be easily tuned. After subsequent carbon coating via pyrolysis of acetylene, the as-formed Ge@C nanocomposite with hollow interiors exhibits a highly reversible capacity of about 920 mA h g(-1) at 200 mA g(-1) over 50 cycles, and excellent rate capability. The small size and the high structural integrity of hollow Ge@C structures contribute to the superior lithium-storage performances.

Chemical synthesis of germanium nanoparticles with uniform size as anode materials for lithium ion batteries

A simple Mg-thermal reduction reaction is reported to synthesize germanium (Ge) nanoparticles with a uniform size at a low temperature of 400 °C in an autoclave.

Thick solid electrolyte interphases grown on silicon nanocone anodes during slow cycling and their negative effects on the performance of Li-ion batteries

Thickness, homogeneity and coverage of the surface passivation layer on Si anodes for Li-ion batteries have decisive influences on their cyclic performance and coulombic efficiency, but related information is difficult to obtain, especially during cycling. In this work, a well-defined silicon nanocone (SNC) on silicon wafer sample has been fabricated as a model electrode in lithium ion batteries to investigate the growth of surface species on the SNC electrode during cycling using ex situ scanning electronic microscopy. It is observed that an extra 5 μm thick layer covers the top of the SNCs after 25 cycles at 0.1 C. This top layer has been proven to be a solid electrolyte interphase (SEI) layer by designing a solid lithium battery. It is noticed that the SEI layer is much thinner at a high rate of 1 C. The cyclic performance of the SNCs at 1 C looks much better than that of the same electrode at 0.1 C in the half cell. Our findings clearly demonstrate that the formation of the thick SEI on the naked nanostructured Si anode during low rate cycling is a serious problem for practical applications. An in depth understanding of this problem may provide valuable guidance in designing Si-based anode materials.

High yield fabrication of hollow vesica-like silicon based on the Kirkendall effect and its application to energy storage

Recently, a unique process based on the Kirkendall effect was employed to generate hollow nanostructures with a wide variety of materials. However, a similar hollow structure of silicon based on the fabrication mechanism of the Kirkendall effect is still not proposed. Here, we provide an extensible synthesis method for the high yield fabrication of a uniform vesica-like hollow Si material from SiO2 based on the Kirkendall effect in a molten salt reduction process. Significantly, without further modification, the as-prepared hollow vesica-like Si exhibits a high electrochemical storage capacity and long cycling properties (∼712 mA h g(-1) at 0.36 A g(-1) over 200 cycles).

Self-assembled asymmetric membrane containing micron-size germanium for high capacity lithium ion batteries

Asymmetric porous structure formedviaa self-assembly phase inversion method can significantly improve the cycling performance of lithium ion anodes made of micron-size germanium powders.

Recent progress in theoretical and computational investigations of Li-ion battery materials and electrolytes

Advancements and progress in computational and theoretical investigations of Li-ion battery materials and electrolytes are reviewed and assessed.

In situ encapsulation of germanium clusters in carbon nanofibers: high-performance anodes for lithium-ion batteries

Alloyed anode materials for lithium-ion batteries (LIBs) usually suffer from considerable capacity losses during charge-discharge process. Herein, in situ-grown germanium clusters are homogeneously encapsulated into porous nitrogen-doped carbon nanofibers (N-CNFs) to form Ge/N-CNFs hybrids, using a facile electrospinning method followed by thermal treatment. When used as anode in LIBs, the Ge/N-CNFs hybrids exhibit excellent lithium storage performance in terms of specific capacity, cycling stability, and rate capability. The excellent electrochemical properties can be attributed to the unique structural features: the distribution of the germanium clusters, porous carbon nanofibers, and GeN chemical bonds all contribute to alleviating the large volume changes of germanium during the discharge-charge process, while at same time the unique porous N-CNFs not only increase the contact area between the electrode and the electrolyte, but also the conductivity of the hybrid.

Bending-induced symmetry breaking of lithiation in germanium nanowires

From signal transduction of living cells to oxidation and corrosion of metals, mechanical stress intimately couples with chemical reactions, regulating these biological and physiochemical processes. The coupled effect is particularly evident in the electrochemical lithiation/delithiation cycling of high-capacity electrodes, such as silicon (Si), where on the one hand lithiation-generated stress mediates lithiation kinetics and on the other the electrochemical reaction rate regulates stress generation and mechanical failure of the electrodes. Here we report for the first time the evidence on the controlled lithiation in germanium nanowires (GeNWs) through external bending. Contrary to the symmetric core-shell lithiation in free-standing GeNWs, we show bending the GeNWs breaks the lithiation symmetry, speeding up lithaition at the tensile side while slowing down at the compressive side of the GeNWs. The bending-induced symmetry breaking of lithiation in GeNWs is further corroborated by chemomechanical modeling. In the light of the coupled effect between lithiation kinetics and mechanical stress in the electrochemical cycling, our findings shed light on strain/stress engineering of durable high-rate electrodes and energy harvesting through mechanical motion.

Vertically aligned CNT-supported thick Ge films as high-performance 3D anodes for lithium ion batteries

The electrochemical performance of a thick Ge film (ca. 1020 nm) is dramatically improved by adopting vertically aligned carbon nanotube (VACNT) arrays as a 3D current collector. The VACNT-supported thick Ge film exhibits high reversible specific capacity (1352 mAh g(-1) ), and excellent capacity retention (97.2% after 100 cycles) and rate capability (843 mAh g(-1) at 10 C).

Four-layer tin-carbon nanotube yolk-shell materials for high-performance lithium-ion batteries

All high-capacity anodes for lithium-ion (Li-ion) batteries, such as those based on tin (Sn) and silicon (Si), suffer from large volume changes during cycling with lithium ions, and their high capacities can be only achieved in the first few cycles. We design and synthesize a unique four-layer yolk-shell tin-carbon (Sn-C) nanotube array to address this problem. The shape and size of the exterior Sn nanotube@carbon core-shell layer, the encapsulated interior Sn nanowire@carbon nanotube core-shell layer, and the filling level of each layer can be all controlled by adjusting the experimental conditions. Such a nanostructure has not been reported for any metal or metal oxide-based material. Owing to the special design of the electrode structure, the four-layer hierarchical structure demonstrates excellent Li-ion storage properties in terms of high capacity, long cycle life, and high rate performance.

High-performance germanium nanowire-based lithium-ion battery anodes extending over 1000 cycles through in situ formation of a continuous porous network

Here we report the formation of high-performance and high-capacity lithium-ion battery anodes from high-density germanium nanowire arrays grown directly from the current collector. The anodes retain capacities of ∼ 900 mAh/g after 1100 cycles with excellent rate performance characteristics, even at very high discharge rates of 20-100C. We show by an ex situ high-resolution transmission electron microscopy and high-resolution scanning electron microscopy study that this performance can be attributed to the complete restructuring of the nanowires that occurs within the first 100 cycles to form a continuous porous network that is mechanically robust. Once formed, this restructured anode retains a remarkably stable capacity with a drop of only 0.01% per cycle thereafter. As this approach encompasses a low energy processing method where all the material is electrochemically active and binder free, the extended cycle life and rate performance characteristics demonstrated makes these anodes highly attractive for the most demanding lithium-ion applications such as long-range battery electric vehicles.

Thermoelectric metal tellurides with nanotubular structures synthesized by the Kirkendall effect and their reduced thermal conductivities

Polycrystalline nanotubular Bi₂Te₃ and PbTe synthesized by the Kirkendall effect showed great decrease in thermal conductivities.

Catalyst-free direct growth of a single to a few layers of graphene on a germanium nanowire for the anode material of a lithium battery

Direct growth of a single to a few layers of graphene on a germanium nanowire (Gr/Ge NW; see picture) was achieved by a metal-catalyst-free chemical vapor deposition (CVD) process. The Gr/Ge NW was used as anode in a lithium ion battery. This material has a specific capacity of 1059 mA h g(-1) at 4.0 C, a long cycle life over 200 cycles, and a high capacity retention of 90%.

Highly conductive and strain-released hybrid multilayer Ge/Ti nanomembranes with enhanced lithium-ion-storage capability

Highly conductive and hybridized microtubes relying on strain-released ultrathin Ti/Ge bilayer nanomembranes are reported. These hybrid multilayer microtubes show a remarkably enhanced reversible capacity up to 1495 mA h g(-1) with a high first-cycle Coulombic efficiency of 85%, and demonstrate an excellent capacity of ≈930 mA h g(-1) after 100 cycles.

Hierarchically porous germanium-modified carbon materials with enhanced lithium storage performance

In this work, hierarchically porous germanium-modified carbon materials (C/Ge) have been successfully synthesized by a facile hydrothermal method followed with a subsequent annealing treatment. The C/Ge nanocomposites have a unique hierarchically microporous-mesoporous structure, with a surface area of 331 m(2) g(-1). The C/Ge composites exhibit improved capacity, cycling performance and rate capability when used as an anode material, compared with the unmodified carbon and commercial germanium. This superior electrochemical performance could be ascribed to two points. On the one hand, such a hierarchically porous carbon would be beneficial to store and insert the lithium, and therefore the pore-transport system would allow the accessibility of those sites by lithium ions. At the same time, the carbon layers can effectively increase the electrode conductivity, and serve as a buffer to accommodate the volume changes of electrode materials during cycling. On the other hand, the Ge also contributes to the enhancement of the capacity of the carbon spheres since Ge is also a promising anode material with large theoretical specific capacity (ca. 1400 mA h g(-1)) for lithium-ion batteries.

Template-free preparation of crystalline Ge nanowire film electrodes via an electrochemical liquid-liquid-solid process in water at ambient pressure and temperature for energy storage

The direct electrodeposition of crystalline germanium (Ge) nanowire film electrodes from an aqueous solution of dissolved GeO(2) using discrete 'flux' nanoparticles capable of dissolving Ge(s) has been demonstrated. Electrodeposition of Ge at inert electrode substrates decorated with small (<100 nm), discrete indium (In) nanoparticles resulted in crystalline Ge nanowire films with definable nanowire diameters and densities without the need for a physical or chemical template. The Ge nanowires exhibited strong polycrystalline character as-deposited, with approximate crystallite dimensions of 20 nm and a mixed orientation of the crystallites along the length of the nanowire. Energy dispersive spectroscopic elemental mapping of individual Ge nanowires showed that the In nanoparticles remained at the base of each nanowire, indicating good electrical communication between the Ge nanowire and the underlying conductive support. As-deposited Ge nanowire films prepared on Cu supports were used without further processing as Li(+) battery anodes. Cycling studies performed at 1 C (1624 mA g(-1)) indicated the native Ge nanowire films supported stable discharge capacities at the level of 973 mA h g(-1), higher than analogous Ge nanowire film electrodes prepared through an energy-intensive vapor-liquid-solid nanowire growth process. The cumulative data show that ec-LLS is a viable method for directly preparing a functional, high-activity nanomaterials-based device component. The work presented here is a step toward the realization of simple processes that make fully functional energy conversion/storage technologies based on crystalline inorganic semiconductors entirely through benchtop, aqueous chemistry and electrochemistry without time- or energy-intensive process steps.