Self-Assembly of Silicon Nanotubes Driven by a Biphasic Transition from the Natural Mineral Montmorillonite in Molten Salt Electrolysis

Silicon nanotubes (SNTs) have been considered as promising anode materials for lithium-ion batteries (LIBs). However, the reported strategies for preparing SNTs generally have special requirements for either expensive templates or complex catalysts. It is necessary to explore a cost-effective and efficient approach for the preparation of high-performance SNTs. In this work, a biphasic transformation strategy involving "solid-state reduction" and "dissolution-deposition" in molten salts is developed to prepare SNTs using montmorillonite as a precursor. The rod-like intermediate of silicon-aluminum-calcium is initially reduced in solid state, which then triggers the continuous dissolution and deposition of calcium silicate in the inner space of the intermediate to form a hollow structure during the subsequent reduction process. The transition from solid to liquid is crucial for improving the kinetics of deoxygenation and induces the self-assembly of SNTs during electrolysis. When the obtained SNTs is used as anode materials for LIBs, they exhibit a high capacity of 2791 mAh g-1 at 0.2 A g-1, excellent rate capability of 1427 mA h g-1 at 2 A g-1, and stable cycling performance with a capacity of 2045 mAh g-1 after 200 cycles at 0.5 A g-1. This work provides a self-assembling, controllable, and cost-effective approach for fabricating SNTs.

Silver-Assisted Chemical Etching for the Fabrication of Porous Silicon N-Doped Nanohollow Carbon Spheres Composite Anodes to Enhance Electrochemical Performance

Silicon (Si) shows great potential as an anode material for lithium-ion batteries. However, it experiences significant expansion in volume as it undergoes the charging and discharging cycles, presenting challenges for practical implementation. Nanostructured Si has emerged as a viable solution to address these challenges. However, it requires a complex preparation process and high costs. In order to explore the above problems, this study devised an innovative approach to create Si/C composite anodes: micron-porous silicon (p-Si) was synthesized at low cost at a lower silver ion concentration, and then porous silicon-coated carbon (p-Si@C) composites were prepared by compositing nanohollow carbon spheres with porous silicon, which had good electrochemical properties. The initial coulombic efficiency of the composite was 76.51%. After undergoing 250 cycles at a current density of 0.2 A·g-1, the composites exhibited a capacity of 1008.84 mAh·g-1. Even when subjected to a current density of 1 A·g-1, the composites sustained a discharge capacity of 485.93 mAh·g-1 even after completing 1000 cycles. The employment of micron-structured p-Si improves cycling stability, which is primarily due to the porous space it provides. This porous structure helps alleviate the mechanical stress caused by volume expansion and prevents Si particles from detaching from the electrodes. The increased surface area facilitates a longer pathway for lithium-ion transport, thereby encouraging a more even distribution of lithium ions and mitigating the localized expansion of Si particles during cycling. Additionally, when Si particles expand, the hollow carbon nanospheres are capable of absorbing the resulting stress, thus preventing the electrode from cracking. The as-prepared p-Si utilizing metal-assisted chemical etching holds promising prospects as an anode material for lithium-ion batteries.

Three-Dimensional Self-Supported Ge Anode for Advanced Lithium-Ion Batteries

Three-dimensional (3D) self-supported Ge anode is one of the promising candidates to replace the traditional graphite anode material for high-performance binder-free lithium-ion batteries (LIBs). The enlarged surface area and the shortened ions/electrons transporting distance of the 3D electrode would greatly facilitate the rapid transfer of abundant lithium ions during cycling, thus achieve enhanced energy and power density during cycling. Cycle stability of the 3D self-supported Ge electrode would be improved due to the obtained enough space could effectively accommodate the large volume expansion of the Ge anode. In this review, we first describe the electrochemical properties and Li ions storage mechanism of Ge anode. Moreover, the recent advances in the 3D self-supported Ge anode architectures design are majorly illustrated and discussed. Challenges and prospects of the 3D self-supported Ge electrode are finally provided, which shed light on ways to design more reliable 3D Ge-based electrodes in energy storage systems.

Iodine-Ion-Assisted Galvanic Replacement Synthesis of Bismuth Nanotubes for Ultrafast and Ultrastable Sodium Storage

Bismuth (Bi) has emerged as a promising anode material for fast-charging and long-cycling sodium-ion batteries (SIBs). However, its dramatically volumetric variations during cycling will undesirably cause the pulverization of active materials, severely limiting the electrochemical performance of Bi-based electrodes. Constructing hollow nanostructures is recognized as an effective way to resolve the volume expansion issues of alloy-type anodes but remains a great challenge for metallic bismuth. Here, we report a facile iodine-ion-assisted galvanic replacement approach for the synthesis of Bi nanotubes (NTs) for high-rate, long-term and high-capacity sodium storage. The hollow tubular structure effectively alleviates the structural strain during sodiation/desodiation processes, resulting in excellent structural stability; the thin wall and large surface area enable ultrafast sodium ion transport. Benefiting from the structural merits, the Bi NT electrode exhibits extraordinary rate capability (84% capacity retention at 150 A g^-1) and outstanding cycling stability (74% capacity retention for 65,000 cycles at 50 A g^-1), which represent the best rate performance and longest cycle life among all reported anodes for SIBs. Moreover, when coupled with the Na₃(VOPO₄)₂F cathode in full cells, this electrode also demonstrates excellent cycling performance, showing the great promise of Bi NTs for practical application. A combination of advanced research techniques reveals that the excellent performance originates from the structural robustness of the Bi NTs and the fast electrochemical kinetics during cycling.

Mesoporous multi-shelled hollow resin nanospheres with ultralow thermal conductivity

Hollow nanostructures exhibit enclosed or semi-enclosed spaces inside and the consequent features of restricting molecular motion, which is crucial for intrinsic physicochemical properties. Herein, we developed a new configuration of hollow nanostructures with more than three layers of shells and simultaneously integrated mesopores on every shell. The novel interior configuration expresses the characteristics of periodic interfaces and abundant mesopores. Benefiting from the suppression of gas molecule convection by boundary scattering, the thermal conductivity of mesoporous multi-shelled hollow resin nanospheres reaches 0.013 W m-1 K-1 at 298 K. The designed interior mesostructural configuration of hollow nanostructures provides an ideal platform to clarify the influence of nanostructure design on intrinsic physicochemical properties and propels the development of hollow nanostructures.

Germanium Nanowires via Molten-Salt Electrolysis for Lithium Battery Anode

Germanium (Ge)-based materials can serve as promising anode candidates for high-energy lithium-ion batteries (LIBs). However, the rapid capacity decay caused by huge volume expansion severely retards their application. Herein, we report a facile and controllable synthesis of Ge nanowire anode materials through molten-salt electrolysis. The optimal Ge nanowires can deliver a capacity of 1058.9 mAh g^-1 at 300 mA g^-1 and a capacity above 602.5 mAh g^-1 at 3000 mA g^-1 for 900 cycles. By in situ transmission electron microscopy and in situ X-ray diffraction, the multiple-step phase transformation and good structural reversibility of the Ge nanowires during charge/discharge are elucidated. When coupled with a lithium-rich Li_1.2Mn_0.567Ni_0.167Co_0.067O₂ cathode in a full battery, the Ge nanowire anode leads to a relatively stable capacity with a retention of 84.5% over 100 cycles. This research highlights the significance of molten-salt electrolysis for the synthesis of alloy-type anode materials toward high-energy LIBs.

Silicate-Mediated Electrolytic Silicon Nanotube from Silica in Molten Salts

Electrochemical preparation/processing of functional silicon from abundant silica is an ideal protocol to promote sustainability of energy sectors. Such an imperative task is challenged by poor electrical conductivity of Si/SiO₂ and inadequate mass diffusion of bulk silicon. Herein, a template-free and one-step preparation of silicon nanotubes (Si-NT) from electrochemical reduction of silica in molten salts is reported. An in situ oriented growth of silicate nanorods (NRs) from silica is clarified as the key step to direct a self-template evolution of Si-NT from silica. The silicate-mediated construction of Si-based NT is versatile and successfully extended to prepare Si-NT/graphite and Ti_x Si_y -NT. Benefitting from fast longitudinal motorways for fast transport of electrons and ions along/inside NTs, an energy consumption 30% lower than industrial silicon production is achieved. When evaluated as anode of lithium ion battery, the Si-NT-based electrodes show outstanding initial Coulombic efficiency and high reversible capacity. The silicate-mediated construction of Si-based NT integrates straightforward preparation and intriguing functionality of Si-based materials, bridging a closed-loop Si-based energy infrastructure.

Carbon Nanotubes Interconnected NiCo Layered Double Hydroxide Rhombic Dodecahedral Nanocages for Efficient Oxygen Evolution Reaction

Proper control of a 3d transition metal-based catalyst with advanced structures toward oxygen evolution reaction (OER) with a more feasible synthesis strategy is of great significance for sustainable energy-related devices. Herein, carbon nanotube interconnected NiCo layered double hydroxide rhombic dodecahedral nanocages (NiCo-LDH RDC@CNTs) were developed here with the assistance of a feasible zeolitic imidazolate framework (ZIF) self-sacrificing template strategy as a highly efficient OER electrocatalyst. Profited by the well-fined rhombic dodecahedral nanocage architecture, CNTs’ interconnected characteristic and structural feature of the vertically aligned nanosheets, the as-synthesized NiCo-LDH RDC@CNTs integrated large exposed active surface areas, enhanced electron transfer capacity and multidimensional mass diffusion channels, and thereby collaboratively afforded the remarkable electrocatalytic performance of the OER. Specifically, the designed NiCo-LDH RDC@CNTs exhibited a distinguished OER activity, which only required a low overpotential of 255 mV to reach a current density of 10 mA cm⁻² for the OER. For the stability, no obvious current attenuation was detected, even after continuous operation for more than 27 h. We certainly believe that the current extraordinary OER activity combined with the robust stability of NiCo-LDH RDC@CNTs enables it to be a great candidate electrocatalyst for economical and sustainable energy-related devices.

Recent Advancement of the Current Aspects of g-C3 N4 for its Photocatalytic Applications in Sustainable Energy System

Being one of the foremost enticing and intriguing innovations, heterogeneous photocatalysis has also been used to effectively gather, transform, and conserve sustainable sun's radiation for the production of efficient and clean fossil energy as well as a wide range of ecological implications. The generation of solar fuel-based water splitting and CO₂ photoreduction is excellent for generating alternative resources and reducing global warming. Developing an inexpensive photocatalyst can effectively split water into hydrogen (H₂ ), oxygen (O₂ ) sources, and carbon dioxide (CO₂ ) into fuel sources, which is a crucial problem in photocatalysis. The metal-free g-C₃ N₄ photocatalyst has a high solar fuel generation potential. This review covers the most recent advancements in g-C₃ N₄ preparation, including innovative design concepts and new synthesis methods, and novel ideas for expanding the light absorption of pure g-C₃ N₄ for photocatalytic application. Similarly, the main issue concerning research and prospects in photocatalysts based g-C₃ N₄ was also discussed. The current dissertation provides an overview of comprehensive understanding of the exploitation of the extraordinary systemic and characteristics, as well as the fabrication processes and uses of g-C₃ N₄ .

Polyacrylic acid and β-cyclodextrin polymer cross-linking binders to enhance capacity performance of silicon/carbon composite electrodes in lithium-ion batteries

Binders play a key role in maintaining the integrity of high-capacity silicon anodes, which otherwise experience serious capacity decay during cycling caused by huge volume variation of the silicon. With an aim to developing a highly efficient polymeric binder to mitigate this capacity decay, we present a novel binder synthesized from polyacrylic acid (PAA) and polymerized β-cyclodextrin (β-CDp) for Si anodes for the lithium-ion batteries. This PAA-β-CDp binder has a 3D network structure, which provides strong adhesion between the active material and the current collector. PAA-β-CDp binder makes silicon anode achieve a specific capacity of 2326.4 mAhg^-1 at the current density of 0.2 A g^-1 with a capacity retention of 64.6% after 100 cycles. The experimental results show that the PAA-β-CDp binder can effectively mitigate the huge volume change and improve the capacity and cycling performance of Si anodes.

Microscale Silicon-Based Anodes: Fundamental Understanding and Industrial Prospects for Practical High-Energy Lithium-Ion Batteries

To accelerate the commercial implementation of high-energy batteries, recent research thrusts have turned to the practicality of Si-based electrodes. Although numerous nanostructured Si-based materials with exceptional performance have been reported in the past 20 years, the practical development of high-energy Si-based batteries has been beset by the bias between industrial application with gravimetrical energy shortages and scientific research with volumetric limits. In this context, the microscale design of Si-based anodes with densified microstructure has been deemed as an impactful solution to tackle these critical issues. However, their large-scale application is plagued by inadequate cycling stability. In this review, we present the challenges in Si-based materials design and draw a realistic picture regarding practical electrode engineering. Critical appraisals of recent advances in microscale design of stable Si-based materials are presented, including interfacial tailoring of Si microscale electrode, surface modification of SiO_x microscale electrode, and structural engineering of hierarchical microscale electrode. Thereafter, other practical metrics beyond active material are also explored, such as robust binder design, electrolyte exploration, prelithiation technology, and thick-electrode engineering. Finally, we provide a roadmap starting with material design and ending with the remaining challenges and integrated improvement strategies toward Si-based full cells.

In-situ anodic precipitation process for highly efficient separation of aluminum alloys

Electrorefining process has been widely used to separate and purify metals, but it is limited by deposition potential of the metal itself. Here we report in-situ anodic precipitation (IAP), a modified electrorefining process, to purify aluminium from contaminants that are more reactive. During IAP, the target metals that are more cathodic than aluminium are oxidized at the anode and forced to precipitate out in a low oxidation state. This strategy is fundamentally based on different solubilities of target metal chlorides in the NaAlCl₄ molten salt rather than deposition potential of metals. The results suggest that IAP is able to efficiently and simply separate components of aluminum alloys with fast kinetics and high recovery yields, and it is also a valuable synthetic approach for metal chlorides in low oxidation states.

Traditional electrorefining process is limited by deposition potential of the metal itself. Here, the authors explore an in-situ anodic precipitation process based on different solubility of target metal chlorides that can efficiently separate components of aluminum alloys.

Fabricating Silicon Nanotubes by Electrochemical Exfoliation and Reduction of Layer-Structured CaSiO3 in Molten Salt

Silicon nanotubes (SNTs) are very attractive in the fields of energy, catalysis, and sensors, but a facile template- and/or catalyst-free preparation method is still absent. Herein, we study a controllable and cost-effective approach for preparing SNTs by electrochemically reducing layer-structured calcium silicate (CaSiO₃) in molten CaCl₂/NaCl without any template and catalyst. The underlying mechanism of the SNT formation is uncovered: the layer-structured CaSiO₃ is first electrochemically exfoliated into SiO_x (0 < x < 2) sheets while releasing CaO into the molten salts, and then the SiO_x sheets are electrochemically reduced and simultaneously crimped into SNTs. The diameter (120-312 nm) and wall thickness (∼40 nm) of the SNTs can be tailorable by manipulating the reduction potential between -1.28 and -1.48 V (vs Ag/AgCl). Lastly, the electrolytic SNTs show a high lithium storage capacity of 3737 mAh g^-1 at 0.2 A g^-1, a high rate capability of 1371 mA h g^-1 at 10 A g^-1, and stable cycling with a capacity of 974 mAh g^-1 after 600 cycles at 1 A g^-1. Overall, the template- and catalyst-free electrochemical method provides a straightforward and facile way to prepare SNTs with a brand-new mechanism that can be applied to other tubular structure materials.

A theoretical study on the electronic, structural and optical properties of armchair, zigzag and chiral silicon-germanium nanotubes

In this work we have studied infinite size silicon-germanium alloy nanotubes of several types, armchair, zigzag and chiral, by theoretical analysis based on density functional theory as implemented in the SIESTA code, which utilizes a linear combination of atomic orbitals and a generalized gradient approximation proposed by Perdew, Burke and Ernzerhof (GGA-PBE) for the exchange and correlation energy. The structures were relaxed until the atomic forces were less than 0.0001 eV Å-1. The electronic band structure, density of states and cohesive energy were then computed; the optical calculation was run in between 0 and 6 eV, with a broadening of 0.05 eV. The obtained results exhibit the deformation of the structure on the surface, which seems to be related to its stability. The armchair and zigzag tubes are direct band gap semiconductor materials, while chiral nanotubes shift from indirect to direct bandgap semiconductors, depending on their diameter size. Likewise, the bandgap depends on the diameter of the SiGe nanotubes (SiGeNTs). We have associated the absorption curves and the density of states through Van Hove singularities. In summary, our results on the structural and electronic properties of SiGeNTs elucidate their possible applications in thermoelectrics, photovoltaics and nanoelectronics, while the possibility of associating the absorption curves with the density of states provides a method of characterization.

Rational Design and Engineering of One-Dimensional Hollow Nanostructures for Efficient Electrochemical Energy Storage

The unique structural characteristics of one-dimensional (1D) hollow nanostructures result in intriguing physicochemical properties and wide applications, especially for electrochemical energy storage applications. In this Minireview, we give an overview of recent developments in the rational design and engineering of various kinds of 1D hollow nanostructures with well-designed architectures, structural/compositional complexity, controllable morphologies, and enhanced electrochemical properties for different kinds of electrochemical energy storage applications (i.e. lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, lithium-selenium sulfur batteries, lithium metal anodes, metal-air batteries, supercapacitors). We conclude with prospects on some critical challenges and possible future research directions in this field. It is anticipated that further innovative studies on the structural and compositional design of functional 1D nanostructured electrodes for energy storage applications will be stimulated.

A cobalt-based metal-organic framework and its derived material as sulfur hosts for aluminum-sulfur batteries with the chemical anchoring effect

With the urgent need to explore high-performance electrochemical energy storage systems, rechargeable Al-ion batteries (AIBs) have attracted attention from researchers and engineers due to their traits, such as abundance and safety. Among all the issues waiting to be solved, the development of a reliable positive electrode material with high specific capacity is an absolute priority for the commercialization of AIBs. Sulfur has a natural advantage when used as the active material, and its theoretical specific capacity is as high as 1675 mA h g-1. MOFs and MOF-derived materials have been proved to be promising hosts for Li-S batteries. Herein, we report a novel Al-S battery system employing MOF (ZIF-67) and MOF-derived materials as sulfur host materials. After being chemically combined with sulfur, the composite still maintains its unique well-defined polyhedron morphology. The voltage hysteresis phenomenon is effectively alleviated with the aid of the host matrix. DFT calculations confirm that ZIF-67 and carbonized ZIF-67-700 polyhedrons can act as an anchor point towards sulfur (S8) and polysulfides (Al2S3, Al2S6, Al2S12, and Al2S18), preventing the detrimental dissolution and shuttle effect. These findings can enlighten future researchers regarding Al-S batteries and broaden the application of MOFs in the field of electrochemical energy storage systems.

Facile and Efficient Fabrication of Branched Si@C Anode with Superior Electrochemical Performance in LIBs

One-dimensional Si nanostructures with carbon coating (1D Si@C) show great potential in lithium ion batteries (LIBs) due to small volume expansion and efficient electron transport. However, 1D Si@C anode with large area capacity still suffers from limited cycling stability. Herein, a novel branched Si architecture is fabricated through laser processing and dealloying. The branched Si, composed of both primary and interspaced secondary dendrites with diameters under 100 nm, leads to improved area capacity and cycling stability. By coating a carbon layer, the branched Si@C anode shows gravimetric capacity of 3059 mAh g^-1 (1.14 mAh cm^-2 ). At a higher rate of 3 C, the capacity is 813 mAh g^-1 , which retained 759 mAh g^-1 after 1000 cycles at 1 C. The area capacity is further improved to 1.93 mAh cm^-2 and remained over 92% after 100 cycles with a mass loading of 0.78 mg cm^-2 . Furthermore, the full-cell configuration exhibits energy density of 405 Wh kg^-1 and capacity retention of 91% after 200 cycles. The present study demonstrates that laser-produced dendritic microstructure plays a critical role in the fabrication of the branched Si and the proposed method provides new insights into the fabrication of Si nanostructures with facility and efficiency.

Rational Design and Mechanical Understanding of Three-Dimensional Macro-/Mesoporous Silicon Lithium-Ion Battery Anodes with a Tunable Pore Size and Wall Thickness

Silicon is regarded as one of the most promising next generation lithium-ion battery anodes due to its exceptional theoretical capacity, appropriate voltage profile, and vast abundance. Nevertheless, huge volume expansion and drastic stress generated upon lithiation cause poor cyclic stability. It has been one of the central issues to improve cyclic performance of silicon-based lithium-ion battery anodes. Constructing hierarchical macro-/mesoporous silicon with a tunable pore size and wall thickness is developed to tackle this issue. Rational structure design, controllable synthesis, and theoretical mechanical simulation are combined together to reveal fundamental mechanisms responsible for an improved cyclic performance. A self-templating strategy is applied using Stöber silica particles as a templating agent and precursor coupled with a magnesiothermic reduction process. Systematic variation of the magnesiothermic reduction time allows good control over the structures of the porous silicon. Finite element mechanical simulations on the porous silicon show that an increased pore size and a reduced wall thickness generate less mechanical stress in average along with an extended lithiation state. Besides the mechanical stress, the evolution of strain and displacement of the porous silicon is also elaborated with the finite element simulation.

Stable Interface between a NaCl-AlCl3 Melt and a Liquid Ga Negative Electrode for a Long-Life Stationary Al-Ion Energy Storage Battery

Intermediate temperature NaCl-AlCl₃-based Al-ion batteries are considered as a promising stationary energy storage system due to their low cost, high safety, etc. However, such a cheap electrolyte has a critical feature, i.e., strong corrosion, which results in the short cycle life of the conventional Al-metal anode and also limits the development of the NaCl-AlCl₃-based Al-ion batteries. A noncorrosive electrolyte may be a good choice for addressing the above challenge, while it is difficult to obtain the electrolyte that has advantages of both noncorrosion and low cost. Therefore, here, we report a Ga-metal anode in the affordable NaCl-AlCl₃ electrolyte for constructing a long-life stationary Al-ion energy storage system. This featured liquid metal anode shows good alloying and dealloying processes between metallic Ga and Al, as well as renders superior stability of the interface between the electrolyte and the anode (e.g., smoothly running for over 580 h at 2 mA cm^-2). No-corrosion and no-pulverization problems appear in this novel liquid/liquid interface. Those advantages demonstrate that the liquid Ga-metal anode has a great promise for the improvement of the NaCl-AlCl₃-based Al-ion batteries for large-scale stationary energy storage applications.

An artificial sea urchin with hollow spines: improved mechanical and electrochemical stability in high-capacity Li-Ge batteries

Metallic germanium (Ge) as the anode can deliver a high specific capacity and high rate capability in lithium ion batteries. However, the large volume expansion largely restrains its further application. Herein, we constructed a three-dimensional sea urchin structure consisting of double layered Ge/TiO₂ nanotubes as the spines via a ZnO template-removing method, which displays a capacity as high as 1060 mA h g^-1 over 130 cycles. The robust, hollow oxide backbone serves as a strong support to accommodate the morphological change of Ge while the enhanced electron-transfer kinetics is attributed to the Ge content and the intimate contact between Ge and TiO₂ during charging/discharging, which were confirmed using in situ transmission electronic microscopy observations and first-principles simulations. In addition, a high capacity retention of batteries using this hybrid composite as the anode was also achieved at low temperature.

Hydrogel-derived VPO4/porous carbon framework for enhanced lithium and sodium storage

Vanadium phosphate (VPO₄) is attracting extensive attention because of its advantages of low cost, stable structure and high theoretical capacity. However, similar to other phosphates, VPO₄ suffers from low electrical conductivity and large volume expansion, adversely influencing its electrochemical performance and thus limiting its application as an anode in lithium and sodium ion batteries. Herein, we propose a novel, facile strategy based on the organic-inorganic network of a nanostructured hybrid hydrogel for immobilizing VPO₄ in a hierarchically porous carbon framework (3DHP-VPO₄@C). VPO₄ chemically interacts with the carbon framework via a P-C bond, functioning as a buffer layer to maintain structural stability during charge/discharge cycles. The carbon framework offers an efficient pathway for electron and Li⁺/Na⁺ transport to ensure high electronic conductivity of the electrode. The 3DHP-VPO₄@C anode exhibits excellent lithium and sodium storage performances, and notably high capacities of 957 mA h g^-1 at 0.1 A g^-1 and 345.3 mA h g^-1 at 5 A g^-1 for lithium ion batteries. Full cells consisting of a LiFePO₄ cathode and the 3DHP-VPO₄@C anode also prove to have superior cycling stability and rate performance for LIBs.

Si and Ge-Based Anode Materials for Li-, Na-, and K-Ion Batteries: A Perspective from Structure to Electrochemical Mechanism

Silicon and germanium are among the most promising candidates as anodes for Li-ion batteries, meanwhile their potential application in sodium- and potassium-ion batteries is emerging. The access of their entire potential requires a comprehensive understanding of their electrochemical mechanism. This Review highlights the processes taking place during the alloying reaction of Si and Ge with the alkali ions. Several associated challenges, including the volumetric expansion, particle pulverization, and uncontrolled formation of solid electrolyte interphase layer must be surmounted and different strategies, such as nanostructures and electrode formulation, have been implemented. Additionally, a new approach based on the use of layered Si and Ge-based Zintl phases is presented. The versatility of this new family permits the tuning of their physical and chemical properties for specific applications. For batteries in particular, the layered structure buffers the volume expansion and exhibits an enhanced electronic conductivity, allowing high power applications.

Atomic Pt Promoted N-Doped Carbon as Novel Negative Electrode for Li-Ion Batteries

In this work, platinum single-atom enhanced mushroom-based carbon (Pt₁/MC) materials have been facilely synthesized and served as novel electrode materials in lithium-ion batteries (LIBs). The as-synthesized Pt₁/MC active material shows a uniform dispersion of isolated Pt atoms on an MC support with high specific surface area and large total pore volume. As a negative electrode material for LIBs, the Pt₁/MC exhibits excellent electrochemical properties, which retains a capacity of 846 mA h g^-1 after 800 cycles at 2 A g^-1 and 349 mA h g^-1 (near to the theoretical capacity of graphite) after 6000 cycles at a high current density of 5 A g^-1. The remarkable high capacity and excellent cycling stability can be attributed to their porous nanostructures and atomic-Pt-enhanced lithium-ion storage. Atomic Pt can compound with Li⁺ ions to form a platinum-lithium alloy during the discharge and charge process. Density functional theory (DFT) calculations are performed to verify that the PtLi₅ alloy is the most stable intermedium on the MC substrate, which further enhances the lithiation and delithiation kinetics. This novel perspective is helpful to explore next-generation negative electrode materials with high capacities and good stabilities for LIBs.

Atomic-scale combination of germanium-zinc nanofibers for structural and electrochemical evolution

Alloys are recently receiving considerable attention in the community of rechargeable batteries as possible alternatives to carbonaceous negative electrodes; however, challenges remain for the practical utilization of these materials. Herein, we report the synthesis of germanium-zinc alloy nanofibers through electrospinning and a subsequent calcination step. Evidenced by in situ transmission electron microscopy and electrochemical impedance spectroscopy characterizations, this one-dimensional design possesses unique structures. Both germanium and zinc atoms are homogenously distributed allowing for outstanding electronic conductivity and high available capacity for lithium storage. The as-prepared materials present high rate capability (capacity of ~ 50% at 20 C compared to that at 0.2 C-rate) and cycle retention (73% at 3.0 C-rate) with a retaining capacity of 546 mAh g⁻¹ even after 1000 cycles. When assembled in a full cell, high energy density can be maintained during 400 cycles, which indicates that the current material has the potential to be used in a large-scale energy storage system.

Alloy anode materials are receiving renewed interest. Here the authors show the design of Ge-Zn nanofibers for lithium ion batteries. Featured by a homogeneous composition at the atomic level and other favorable structural attributes, the materials allow for impressive electrochemical performance.

Tunable Core-Shell Nanowire Active Material for High Capacity Li-Ion Battery Anodes Comprised of PECVD Deposited aSi on Directly Grown Ge Nanowires

Herein, we report the formation of core@shell nanowires (NWs) comprised of crystalline germanium NW cores with amorphous silicon shells (Ge@aSi) and their performance as a high capacity Li-ion battery anode material. The Ge NWs were synthesized directly from the current collector in a solvent vapor growth (SVG) system and used as hosts for the deposition of the Si shells via a plasma-enhanced chemical vapor deposition (PECVD) process utilizing an expanding thermal plasma (ETP) source. The secondary deposition allows for the preparation of Ge@aSi core@shell structures with tunable Ge/Si ratios (2:1 and 1:1) and superior gravimetric and areal capacities, relative to pure Ge. The binder-free anodes exhibited discharge capacities of up to 2066 mAh/g and retained capacities of 1455 mAh/g after 150 cycles (for the 1:1 ratio). The 2:1 ratio showed a minimal ∼5% fade in capacity between the 20th and 150th cycles. Ex situ microscopy revealed a complete restructuring of the active material to an interconnected Si_{1- x}Ge _x morphology due to repeated lithiation and delithiation. In full-cell testing, a prelithiation step counteracted first cycle Li consumption and resulted in a 2-fold improvement to the capacity of the prelithiated cell versus the unconditioned full-cells. Remarkable rate capability was also delivered where capacities of 750 mAh/g were observed at a rate of 10 C.

Galvanic Replacement Synthesis of Highly Uniform Sb Nanotubes: Reaction Mechanism and Enhanced Sodium Storage Performance

One-dimensional nanotubes are very useful for achieving excellent performance in lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to the fact that tubular structures can effectively alleviate the structural strain and shorten the ion diffusion length during repeated cycling. In this work, we report a Cu₂Sb-mediated growth strategy to controllably fabricate highly uniform Sb nanotubes (NTs), as well as Cu@Cu₂Sb and Cu₂Sb@Sb composite NTs, via a facile galvanic replacement reaction using Cu nanowires (NWs) as sacrificial templates. Benefiting from their structural merits, the Sb NTs manifest excellent sodium storage performance with superior rate performance (286 mAh g^-1 at 10 A g^-1) and extraordinary cycling stability (342 mAh g^-1 after 6000 cycles at 1 A g^-1). Furthermore, a full cell with Sb NTs as anode and Na₃(VOPO₄)₂F as cathode exhibits a high energy density (252 Wh kg^-1) and high output voltage (2.7 V), revealing their significant application promise in the next-generation SIBs.

Hierarchical mesoporous cobalt silicate architectures as high-performance sulfate-radical-based advanced oxidization catalysts

Self-sacrificial biomass-derived silica is a rising and promising approach to fabricate large metal silicates, which are practical water treatment agents ascribed for easy sedimentation and separation. However, the original biomass architecture is difficult to be maintained and utilized. Furthermore, sufficient ion diffusion pathways need to be created to satisfy massive mass transport in large bulk materials. Herein, a series of metal silicates, including cobalt silicate (CoSiO_x), copper silicate, nickel silicate, iron silicate, and magnesium silicate, are synthesized from Indocalamus tessellatus leaf as the biomass-derived silica source and investigated as catalysts in sulfate-radical-based advanced oxidization processes (SR-AOPs) for the first time. Among them, CoSiO_x presents an analogical sandwich structure as a leaf-derived template of micron-level size. More importantly, the interior hollow nanotubes assembled by small nanosheets provide numerous pathways for ion diffusion and remarkably promote the mass transport in such large bulk materials. Owing to the combination of the unique structure with the high reactivity of Co (II) toward peroxymonosulfate, CoSiO_x exhibits excellent catalytic performance with 0.242 and 0.153 min^-1 rate constants for the removal of methylene blue and phenol, respectively, which outperforms/is comparable to that of the reported nanomaterials toward organic contaminants in SR-AOPs.

In Situ Pyrolysis Concerted Formation of Si/C Hybrids during Molten Salt Electrolysis of SiO2@Polydopamine

Aiming to enhanced productivity and improved functionality of electrolytic silicon from electroreduction of solid silica in molten salts, we herein report a one-pot electrochemical preparation of Si/C hybrids via pyrolysis-cum-electrolysis (PCE) of SiO₂@polydopamine (SiO₂@PDA) in molten NaCl-CaCl₂ at 800 °C. The obtained hybrids, denoted Si@C@Si, are composed of outmost silicon thin layers due to electrodeposition, sandwiched N-doped carbon hollow spheres derived from pyrolysis of PDA, and encapsulated silicon nanoparticles stemming from direct electrodeoxidation of SiO₂. The PCE protocol shows intriguing merits on accelerated electroreduction of SiO₂ and retarded generation of inconvenient SiC. The preparation conditions of Si@C@Si are optimized by varying electrolysis time and applied voltage, with the optimal conditions being identified as PCE at 2.6 V for 2 h. When evaluated as an anode for lithium-ion batteries, the obtained Si@C@Si exhibits a reversible specific capacity of 904 mAh g^-1 after 100 galvanostatic charge/discharge cycles at 500 mA g^-1. The proposed PCE method is highlighted as an intensified Si extraction method for advanced lithium-ion batteries, promising practical applications.

Cr2O3 nanosheet/carbon cloth anode with strong interaction and fast charge transfer for pseudocapacitive energy storage in lithium-ion batteries

Flexible lithium-ion batteries have attracted considerable interest for next-generation bendable, implantable and wearable electronics devices. Here, we have successfully grown Cr₂O₃ nanosheets on carbon cloth (CC) as freestanding anodes for Li-ion batteries (LIBs). Density functional theory (DFT) calculations verify an optimal structure of two-dimensional Cr₂O₃ nanosheets on the carbon fiber surface and a strong interaction between the O edges of Cr₂O₃ and the carbon. The interconnected Cr₂O₃ nanosheets with a large surface area enable fast charge transfer by efficient contact with electrolyte while the flexible CC substrate accommodates the volume change during cycles, leading to excellent rate capability and cyclic stability through psuedocapacitance-dominant energy storage. Full cells are assembled using the Cr₂O₃-CC anode and a LiFePO₄ cathode, which deliver excellent capacity retention and rate capability. The fully-charged cell is demonstrated to power a red light-emitting diode (LED), verifying the potential of Cr₂O₃-CC as a promising anode material for LIBs.

2D Cr₂O₃ nanosheets are grown on CC with strong interaction and fast charge transfer, and exhibit excellent cyclic performance for LIBs.

Yolk-Shell Germanium@Polypyrrole Architecture with Precision Expansion Void Control for Lithium Ion Batteries

The key properties of yolk-shell architecture in improving electrochemical performance lies in its uniformity and the appropriate void space, which can expand/contract freely upon lithium alloying and leaching without damaging the outer shell, while being achievable with minimal sacrifice of volumetric energy density. Therefore, we developed a highly controllable strategy to fabricate a uniform porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture with conformal Al₂O₃ sacrificial layer by atomic layer deposition (ALD) process. The PGe@PPy yolk-shell anode fabricated with 300 ALD cycles delivers excellent electrochemical performance: high reversible capacity (1,220 mA hr g⁻¹), long cycle performance (>95% capacity retention after 1,000 cycles), and excellent rate capability (>750 mA hr g⁻¹ at 32 A g⁻¹). Electrodes with high areal capacity and current density were also successfully fabricated, opening a new pathway to develop high-capacity electrode materials with large volume expansion.

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Porous germanium@polypyrrole (PGe@PPy) yolk-shell architecture was developed

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Precision expansion and void control make PGe@PPy stable during lithiation/delithiation

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PGe@PPy electrode shows high rate and areal capacity, cycling stability, and current density

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The full cell shows the stable capacity retention with high energy density

Axial Si-Ge Heterostructure Nanowires as Lithium-Ion Battery Anodes

Here, we report the application of axially heterostructured nanowires consisting of alternating segments of silicon and germanium with a tin seed as lithium-ion battery anodes. During repeated lithiation and delithiation, the heterostructures completely rearrange into a porous network of homogeneously alloyed Si_{1- x}Ge _x ligaments. The transformation was characterized through ex situ TEM, STEM, and Raman spectroscopy. Electrochemical analysis was conducted on the heterostructure nanowires with discharge capacities in excess of 1180 mAh/g for 400 cycles (C/5) and capacities of up to 613 mAh/g exhibited at a rate of 10 C.

The Design and Synthesis of Hollow Micro-/Nanostructures: Present and Future Trends

Hollow micro-/nanostructures have attracted tremendous interest owing to their intriguing structure-induced physicochemical properties and great potential for widespread applications. With the development of modern synthetic methodology and analytical instruments, a rapid structural/compositional evolution of hollow structures from simple to complex has occurred in recent decades. Here, an updated overview of research progress made in the synthesis of hollow structures is provided. After an introduction of definition and classification, achievements in synthetic approaches for these delicate hollow architectures are presented in detail. According to formation mechanisms, these strategies can be categorized into four different types, including hard-templating, soft-templating, self-templated, and template-free methods. In particular, the rationales and emerging innovations in conventional templating syntheses are in focus. The development of burgeoning self-templating strategies based on controlled etching, outward diffusion, and heterogeneous contraction is also summarized. In addition, a brief overview of template-free methods and recent advances on combined mechanisms is provided. Notably, the strengths and weaknesses of each category are discussed in detail. In conclusion, a perspective on future trends in the research of hollow micro-/nanostructures is given.

Metal Organic Framework Derived Materials: Progress and Prospects for the Energy Conversion and Storage

Exploring new materials with high efficiency and durability is the major requirement in the field of sustainable energy conversion and storage systems. Numerous techniques have been developed in last three decades to enhance the efficiency of the catalyst systems, control over the composition, structure, surface area, pore size, and moreover morphology of the particles. In this respect, metal organic framework (MOF) derived catalysts are emerged as the finest materials with tunable properties and activities for the energy conversion and storage. Recently, several nano- or microstructures of metal oxides, chalcogenides, phosphides, nitrides, carbides, alloys, carbon materials, or their hybrids are explored for the electrochemical energy conversion like oxygen evolution, hydrogen evolution, oxygen reduction, or battery materials. Interest on the efficient energy storage system is also growing looking at the practical applications. Though, several reviews are available on the synthesis and application of MOF and MOF derived materials, their applications for the electrochemical energy conversion and storage is totally a new field of research and developed recently. This review focuses on the systematic design of the materials from MOF and control over their inherent properties to enhance the electrochemical performances.

Stress-Tolerant Nanoporous Germanium Nanofibers for Long Cycle Life Lithium Storage with High Structural Stability

Nanowires (NWs) synthesized via chemical vapor deposition (CVD) have demonstrated significant improvement in lithium storage performance along with their outstanding accommodation of large volume changes during the charge/discharge process. Nevertheless, NW electrodes have been confined to the research level due to the lack of scalability and severe side reactions by their high surface area. Here, we present nanoporous Ge nanofibers (NPGeNFs) having moderate nanoporosity via a combination of simple electrospinning and a low-energetic zincothermic reduction reaction. In contrast with the CVD-assisted NW growth, our method provides high tunability of macro/microscopic morphologies such as a porosity, length, and diameter of the nanoscale 1D structures. Significantly, the customized NPGeNFs showed a highly suppressed volume expansion of less than 15% (for electrodes) after full lithation and excellent durability with high lithium storage performance over 500 cycles. Our approach offers effective 1D nanostructuring with highly customized geometries and can be extended to other applications including optoelectronics, catalysis, and energy conversion.

Synthesis and Investigation of CuGeO3 Nanowires as Anode Materials for Advanced Sodium-Ion Batteries

Germanium is considered as a potential anode material for sodium-ion batteries due to its fascinating theoretical specific capacity. However, its poor cyclability resulted from the sluggish kinetics and large volume change during repeated charge/discharge poses major threats for its further development. One solution is using its ternary compound as an alternative to improve the cycling stability. Here, high-purity CuGeO₃ nanowires were prepared via a facile hydrothermal method, and their sodium storage performances were firstly explored. The as-obtained CuGeO₃ delivered an initial charge capacity of 306.7 mAh g^-1 along with favorable cycling performance, displaying great promise as a potential anode material for sodium ion batteries.

Exploring Critical Factors Affecting Strain Distribution in 1D Silicon-Based Nanostructures for Lithium-Ion Battery Anodes

Despite the advantage of high capacity, the practical use of the silicon anode is still hindered by large volume expansion during the severe pulverization lithiation process, which results in electrical contact loss and rapid capacity fading. Here, a combined electrochemical and computational study on the factor for accommodating volume expansion of silicon-based anodes is shown. 1D silicon-based nanostructures with different internal spaces to explore the effect of spatial ratio of voids and their distribution degree inside the fibers on structural stability are designed. Notably, lotus-root-type silicon nanowires with locally distributed void spaces can improve capacity retention and structural integrity with minimum silicon pulverization during lithium insertion and extraction. The findings of this study indicate that the distribution of buffer spaces, electrochemical surface area, as well as Li diffusion property significantly influence cycle performance and rate capability of the battery, which can be extended to other silicon-based anodes to overcome large volume expansion.