Formation of Si Hollow Structures as Promising Anode Materials through Reduction of Silica in AlCl3-NaCl Molten Salt

Recent progress in Si/Ti3C2Tx MXene anode materials for lithium-ion batteries

Cardiovascular diseases (CVDs) are a major global health issue, causing significant morbidity and mortality worldwide. Early diagnosis and continuous monitoring of physiological signals are crucial for managing cardiovascular diseases, necessitating the development of lightweight and cost-effective wearable devices. These devices should incorporate portable energy storage systems, such as lithium-ion batteries (LIBs). To enhance the durability and consistency of the monitoring systems, there is a need to develop LIBs with high energy density. Silicon-based materials hold great promise for future LIBs anodes due to their high theoretical capacity and cost-efficiency. Despite their potential, silicon-based materials encounter challenges like substantial volume fluctuations and sluggish kinetics. Transition metal carbide, MXene, features a two-dimensional structure, offering advantages in silicon-based anode materials. This review initially presents the potential of silicon-based anodes and then addresses their challenges. Subsequently, the advantages of MXene are systematically reviewed, including unique structure, abundant surface functional groups, excellent electrical conductivity, and excellent ion transport performance. Next, the detailed discussion covers recent advancements in Si/Ti3C2Tx MXene anode materials for LIBs, with a focus on their synthesis methods. Finally, the challenges and future perspectives of synthesizing Si/Ti3C2Tx nanocomposites are examined, aiming to provide a foundational resource for designing advanced materials for high-energy LIBs.

Simple Solution Plasma Synthesis of Ni@NiO as High-Performance Anode Material for Lithium-Ion Batteries Application

Pursuing of straightforward and cost-effective methods for synthesizing high-performance anode materials for lithium-ion batteries is a topic of significant interest. This study elucidates a one-step synthesis approach for a conversion composite using glow discharge in a nickel formate solution, yielding a composite precursor comprising metallic nickel, nickel hydroxide, and basic nickel salts. Subsequent annealing of the precursor facilitated the formation of the Ni@NiO composite, exhibiting exceptional electrochemical properties as anode material in Li-ion batteries: a capacity of approximately 1000 mAh g-1, cyclic stability exceeding 100 cycles, and favorable rate performance (200 mAh g-1 at 10 A g Collapse

Key Words

• Anode material
• Composite Ni@NiO
• Conversion metal oxide anodes
• Lithium-ion batteries
• One-step plasma solution synthesis

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Grants

• 20-53-04010 RFBR
• F21RM-104 RFBR

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Affiliation(s)

Evgenii Beletskii Institute of Chemistry, St. Petersburg University, St. Petersburg, Universitetskaya Emb.7/9, 199034, Russia
Mikhail Pinchuk Institute for Electrophysics and Electrical Power of the Russian Academy of Sciences, Dvortsovaya Naberezhnaya 18, St. Petersburg, 191186, Russia
Vadim Snetov Institute for Electrophysics and Electrical Power of the Russian Academy of Sciences, Dvortsovaya Naberezhnaya 18, St. Petersburg, 191186, Russia
Aleksandr Dyachenko Institute for Electrophysics and Electrical Power of the Russian Academy of Sciences, Dvortsovaya Naberezhnaya 18, St. Petersburg, 191186, Russia
Alexey Volkov Institute of Chemistry, St. Petersburg University, St. Petersburg, Universitetskaya Emb.7/9, 199034, Russia
Egor Savelev Institute of Chemistry, St. Petersburg University, St. Petersburg, Universitetskaya Emb.7/9, 199034, Russia
Valentin Romanovski Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA, 22904, USA

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Lithium Hydride in the Solid Electrolyte Interphase of Lithium-Ion Batteries as a Pulverization Accelerator of Silicon

As a highly promising next-generation high-specific capacity anode, the industrial-scale utilization of micron silicon has been hindered by the issue of pulverization during cycling. Although numerous studies have demonstrated the effectiveness of regulating the inorganic components of the solid electrolyte interphase (SEI) in improving pulverization, the evolution of most key inorganic components in the SEI and their correlation with silicon failure mechanisms remain ambiguous. This study provides a clear and direct correlation between the lithium hydride (LiH) in the SEI and the degree of micron silicon pulverization in the battery system. The reverse lithiation behavior of LiH on micron silicon during de-lithiation exacerbates the localized stress in silicon particles and contributes to particle pulverization. This work successfully proposes a novel approach to decouple the SEI from electrochemical performance, which can be significant to decipher the evolution of critical SEI components at varied battery anode interfaces and analyze their corresponding failure mechanisms.

Rapid Release of Silicon by Ultrafast Joule Heating Generates Mechanically Stable Shell-Shell Si/C Anodes with Dominant Inward Deformation

As a promising anode material, silicon-carbon composites encounter great challenges related to internal stress release and contact between the composites during lithiation. These issues lead to material degradation and concomitantly rapid capacity decline. Here, we report a type of shell-shell silicon-carbon (SS-Si/C) composite, which consists of a carbon shell tightly coated with a silicon shell. The mechanical analysis unveils that the dominant inward expansion of the Si shell is achieved through the synergistic effect of the outer carbon shell and the inner hollow structure. Benefiting from the well-tailored shell-shell structure, the SS-Si/C anode exhibits exceptional performance, boasting a high specific capacity (1690.3 mA h g-1 after 550 cycles at 0.5 A g-1), a high areal capacity (2.05 mA h cm-2 after more than 400 cycles at 0.5 mA cm-2), and an extended cycling life (1055.6 mA h g-1 after 1000 cycles at 8 A g-1), far exceeding commercially available Si/C anodes. Using the well-designed SS-Si/C anode, full cells assembled with LiCoO2 (LCO) or LiFePO4 (LFP) cathodes achieve favorable rate capability and cyclic stability. Notably, at a high rate of 6 C (1 C = 170 and 270 mA g-1 for LFP and LCO, respectively), these full cells deliver high specific capacities of 79.5 mA h g-1 and 64.9 mA h g-1 when using LCO and LFP, respectively, demonstrating the potential of SS-Si/C anodes for practical applications. The straightforward and safe synthesis method in this work enables the rational design of hollow structures with distinct properties.

Catalytically Active Carbon for Oxygen Reduction Reaction in Energy Conversion: Recent Advances and Future Perspectives

The shortage and unevenness of fossil energy sources are affecting the development and progress of human civilization. The technology of efficiently converting material resources into energy for utilization and storage is attracting the attention of researchers. Environmentally friendly biomass materials are a treasure to drive the development of new-generation energy sources. Electrochemical theory is used to efficiently convert the chemical energy of chemical substances into electrical energy. In recent years, significant progress has been made in the development of green and economical electrocatalysts for oxygen reduction reaction (ORR). Although many reviews have been reported around the application of biomass-derived catalytically active carbon (CAC) catalysts in ORR, these reviews have only selected a single/partial topic (including synthesis and preparation of catalysts from different sources, structural optimization, or performance enhancement methods based on CAC catalysts, and application of biomass-derived CACs) for discussion. There is no review that systematically addresses the latest progress in the synthesis, performance enhancement, and applications related to biomass-derived CAC-based oxygen reduction electrocatalysts synchronously. This review fills the gap by providing a timely and comprehensive review and summary from the following sections: the exposition of the basic catalytic principles of ORR, the summary of the chemical composition and structural properties of various types of biomass, the analysis of traditional and the latest popular biomass-derived CAC synthesis methods and optimization strategies, and the summary of the practical applications of biomass-derived CAC-based oxidative reduction electrocatalysts. This review provides a comprehensive summary of the latest advances to provide research directions and design ideas for the development of catalyst synthesis/optimization and contributes to the industrialization of biomass-derived CAC electrocatalysis and electric energy storage.

Stratum-Confined Solid-State Reaction (SC-SSR) toward Colloidal Silicon-Based Hollow Nanostructures for Bioapplications

Silicon nanostructures (SiNSs) can provide multifaceted bioapplications; but preserving their subhundred nm size during high-temperature silica-to-silicon conversion is the major bottleneck. The SC-SSR utilizes an interior metal-silicide stratum space at a predetermined radial distance inside silica nanosphere to guide the magnesiothermic reduction reaction (MTR)-mediated synthesis of hollow and porous SiNSs. In depth mechanistic study explores solid-to-hollow transformation encompassing predefined radial boundary through the participation of metal-silicide species directing the in-situ formed Si-phase accumulation within the narrow stratum. Evolving thin-porous Si-shell remains well protected by the in-situ segregated MgO emerging as a protective cast against the heat-induced deformation and interparticle sintering. Retrieved hydrophilic SiNSs (<100 nm) can be conveniently processed in different biomedia as colloidal solutions and endocytosized inside cells as photoluminescence (PL)-based bioimaging probes. Inside the cell, rattle-like SiNSs encapsulated with Pd nanocrystals can function as biorthogonal nanoreactors to catalyze intracellular synthesis of probe molecules through C-C cross coupling reaction.

Low Temperature Aluminothermic Reduction of Natural Sepiolite to High-Performance Si Nanofibers for Li-Ion Batteries

Nanostructure silicon is one of the most promising anode materials for the next-generation lithium-ion battery, but the complicated synthesis process and high cost limit its large-scale commercial application. Herein, a simple and low-cost method was proposed to prepare silicon nanofibers (SNF) using natural sepiolite as a template via a low-temperature aluminum reduction process. The low temperature of 260°C during the reduction process not only reduced the production cost but also avoided the destruction of the natural sepiolite structure caused by the high temperature above 600°C in the traditional magnesium thermal reduction process, leading to a more complete nanofiber structure in the final product. For the first time, the important role of Mg-O octahedral structure in the maintenance of nanofiber structure during the process of low-temperature aluminothermic reduction was verified by experiments. When used as an anode for lithium-ion batteries, SNF yield a high reversible capacity of 2005.4 mAh g−1 at 0.5 A g−1 after 50 cycles and 1017.6 mAh g−1 at 2 A g−1 after 200 cycles, remarkably outperforming commercial Si material. With a low-cost precursor and facile approach, this work provides a new strategy for the synthesis of a commercial high-capacity Si anode.

Solid-State Reaction Synthesis of Nanoscale Materials: Strategies and Applications

Nanomaterials (NMs) with unique structures and compositions can give rise to exotic physicochemical properties and applications. Despite the advancement in solution-based methods, scalable access to a wide range of crystal phases and intricate compositions is still challenging. Solid-state reaction (SSR) syntheses have high potential owing to their flexibility toward multielemental phases under feasibly high temperatures and solvent-free conditions as well as their scalability and simplicity. Controlling the nanoscale features through SSRs demands a strategic nanospace-confinement approach due to the risk of heat-induced reshaping and sintering. Here, we describe advanced SSR strategies for NM synthesis, focusing on mechanistic insights, novel nanoscale phenomena, and underlying principles using a series of examples under different categories. After introducing the history of classical SSRs, key theories, and definitions central to the topic, we categorize various modern SSR strategies based on the surrounding solid-state media used for nanostructure growth, conversion, and migration under nanospace or dimensional confinement. This comprehensive review will advance the quest for new materials design, synthesis, and applications.

Advances of Synthesis Methods for Porous Silicon-Based Anode Materials

Silicon (Si)-based anode materials have been the promising candidates to replace commercial graphite, however, there are challenges in the practical applications of Si-based anode materials, including large volume expansion during Li⁺ insertion/deinsertion and low intrinsic conductivity. To address these problems existed for applications, nanostructured silicon materials, especially Si-based materials with three-dimensional (3D) porous structures have received extensive attention due to their unique advantages in accommodating volume expansion, transportation of lithium-ions, and convenient processing. In this review, we mainly summarize different synthesis methods of porous Si-based materials, including template-etching methods and self-assembly methods. Analysis of the strengths and shortages of the different methods is also provided. The morphology evolution and electrochemical effects of the porous structures on Si-based anodes of different methods are highlighted.

One-Dimensional Nanoscale Si/Co Based on Layered Double Hydroxides towards Electrochemical Supercapacitor Electrodes

It is well known that layered double hydroxides (LDHs) are two-dimensional (2D) layered compounds. However, we modified these 2D layered compounds to become one-dimensional (1D) nanostructures destined for high-performance supercapacitors applications. In this direction, silicon was inserted inside the nanolayers of Co-LDHs producing nanofibers of Si/Co LDHs through the intercalation of cyanate anions as pillars for building nanolayered structures. Additionally, nanoparticles were observed by controlling the preparation conditions and the silicon percentage. Scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and thermal analyses have been used to characterize the nanolayered structures of Si/Co LDHs. The electrochemical characterization was performed by cyclic voltammetry and galvanic charge–discharge technique in 2M KOH electrolyte solution using three-electrode cell system. The calculated specific capacitance results indicated that the change of morphology from nanoparticles or plates to nanofibers had a positive effect for improving the performance of specific capacitance of Si/Co LDHs. The specific capacitance enhanced to be 621.5 F g−1 in the case of the nanofiber of Si/Co LDHs. Similarly, the excellent cyclic stability (84.5%) was observed for the nanofiber. These results were explained through the attribute of the nanofibrous morphology and synergistic effects between the electric double layer capacitive character of the silicon and the pseudo capacitance nature of the cobalt. The high capacitance of ternary Si/Co/cyanate LDHs nanocomposites was suggested to be used as active electrode materials for high-performance supercapacitors applications.

Extraction and Synthesis of Silicon Nanoparticles (SiNPs) from Sugarcane Bagasse Ash: A Mini-Review

This current study reviews the utilization of the traditional extraction methods and latest findings in extraction of silica from agricultural wastes, in particular, sugarcane bagasse, using inorganic acids to produce nano-silicon. The three key processes discussed in detail include electrochemical, ball milling, and sol–gel processes. The sugarcane bagasse has been identified as the cheapest source of producing silica from the potential raw material for the preparation of nano-silicon. The acid-base extraction and precipitation methodology involves the use of bases like sodium hydroxide (NaOH) and potassium hydroxide (KOH), and acids such as hydrofluoric acid (HF), sulphuric acid (H2SO4), nitric acid (HNO3), and hydrochloric acid (HCl) for the treatment of the ash. Sugarcane bagasse has notably emerged as an excellent and sustainable source of both tailored silica particles and bioenergy production. The ability to manipulate the engineered silica particles at the nano-level from sugarcane bagasse-based silica is explained in detail. Silica is a significant raw material with various industrial applications, with much research underway to extract it efficiently from industrial agro-waste, such as sugarcane bagasse. The production of highly pure silicon nanoparticles from sugarcane bagasse ash will serve as an important synthetic route in lowering the manufacturing costs and providing a low-cost polycrystalline silicon semiconductor for niche application in thin film solar technology.

Dual carbon and void space confined SiOx/C@void@Si/C yolk-shell nanospheres with high-rate performances and outstanding cyclability for lithium-ion batteries anodes

Silicon-based anode materials with high theoretical capacity have great challenges of enormous volume expansion and poor electronic conductivity. Herein, a novel dual carbon confined SiO_x/C@void@Si/C yolk-shell monodisperse nanosphere with void space have been fabricated through hydrothermal reaction, carbonization, and in-situ low-temperature aluminothermic reduction. Furthermore, the O/Si ratio and void space between SiO_x/C core and Si/C shell can be effectively tuned by the length of aluminothermic reduction time. The SiO_x/C core plays a role of maintaining the spherical structure and the void space can accommodate the volume expansion of Si. Moreover, the inner and outer carbons not only alleviate volume variation of SiO_x and Si but also enhance the electrical conductivity of composites. Benefiting from the synergy of the double carbon and void space, the optimized VSC-14 anode affords prominent cycle stability with reversible capacity of 1094 mAh g^-1 after 550 cycles at 200 mA g^-1. By pre-lithiation treatment, the VSC-14 achieves an initial Coulombic efficiency of 93.27% at 200 mA g^-1 and a reversible capacity of 348 mAh g^-1 at 5 A g^-1 after 4000 cycles. Furthermore, the pouch cell using VSC-14 anode and LiFePO₄ cathode delivers a reversible capacity of 138 mAh g^-1 at 0.2C. We hope this strategy can provide a scientific method to synthesis yolk-shell Si-based materials.

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.

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.

Recent Advances in Silicon-Based Electrodes: From Fundamental Research toward Practical Applications

The increasing demand for higher-energy-density batteries driven by advancements in electric vehicles, hybrid electric vehicles, and portable electronic devices necessitates the development of alternative anode materials with a specific capacity beyond that of traditional graphite anodes. Here, the state-of-the-art developments made in the rational design of Si-based electrodes and their progression toward practical application are presented. First, a comprehensive overview of fundamental electrochemistry and selected critical challenges is given, including their large volume expansion, unstable solid electrolyte interface (SEI) growth, low initial Coulombic efficiency, low areal capacity, and safety issues. Second, the principles of potential solutions including nanoarchitectured construction, surface/interface engineering, novel binder and electrolyte design, and designing the whole electrode for stability are discussed in detail. Third, applications for Si-based anodes beyond LIBs are highlighted, specifically noting their promise in configurations of Li-S batteries and all-solid-state batteries. Fourth, the electrochemical reaction process, structural evolution, and degradation mechanisms are systematically investigated by advanced in situ and operando characterizations. Finally, the future trends and perspectives with an emphasis on commercialization of Si-based electrodes are provided. Si-based anode materials will be key in helping keep up with the demands for higher energy density in the coming decades.

Titanium Monoxide-Stabilized Silicon Nanoparticles with a Litchi-like Structure as an Advanced Anode for Li-ion Batteries

Silicon (Si) has been considered as the most potential anode material for next-generation high-energy density lithium-ion batteries (LIBs) because of its extremely high theoretical capacity. However, the performance deterioration caused by volume change and low electrical conductivity of active Si particles greatly limit its commercial use. Here, we designed a nonstoichiometric TiO_x-coated Si anode with a litchi-like structure, in which Si-Ti and Si-O dual bonds are expected to form between the Si core and TiO_x shell. This unique structure plays a major role in preventing the volume expansion and improving the electrical conductivity of the Si anode. The as-prepared TiO_x-coated Si anode could exhibit excellent cycling stability after 1000 cycles at 1000 mA g^-1 with a relatively small capacity decay rate of ∼0.04% per cycle, which can be comparable to most of the modified Si anodes in references. This strategy of surface regulating on the Si anode could be extended to other electrodes with large volume expansion during cycling in LIBs for achieving competitive electrochemical properties.

Long-Term Stable Hollowed Silicon for Li-Ion Batteries Based on an Improved Low-Temperature Molten Salt Strategy

Nanostructured hollow silicon has attracted tremendous attention as high-performance anode materials in Li-ion battery applications. However, the large-scale production of pure hollowed silicon with long cycling stability is still a great challenge. Here, we report an improved low-temperature molten salt strategy to synthesize nanosized hollowed silicon with a stable structure on a large scale. As an anode material for rechargeable lithium-ion batteries, it exhibits a high capacity, excellent long cycling, and steady rate performance at different current densities. Especially, a high reversible capacity of 2028.6 mA h g^-1 at 0.5 A g^-1 after 150 cycles, 994.3 mA h g^-1 at 3 A g^-1 after 500 cycles, and 538.8 mAh g^-1 at 5 A g^-1 after 1200 cycles could be obtained. This kind of nanosized hollowed silicon can be applied as a basic anode material in silicon-based composites for long-term stable Li-ion battery applications.

Template-Free Electrochemical Formation of Silicon Nanotubes from Silica

Silicon, with its elaborate microstructure, plays important roles in energy materials. In operando engineering of microstructure during extraction is an ideal protocol to develop advanced Si-based materials. A template-free electrochemical preparation of silicon nanotubes (Si-NT) is herein achieved by co-electrolysis of SiO2 and AgCl in molten NaCl-CaCl2 at 850 °C. The in situ electrodeposited Ag facilitates the generation of a liquid Ag-Si intermediate, triggering a liquid-solid mechanism to direct the growth of Si-NT. An automatic separation of Ag from Si then occurs in the following cooling process, resulting in Ag deposits on the Ni current collector and recycling of Ag. Such a facile and smart preparation of Si-NT from affordable silica guarantees an enhanced current efficiency of 74%, a decreased energy consumption of 12.1 kW h kgSi -1, and enhanced lithium-storage capability of the electrolytic Si-NT. An in situ coating of Ag over the Si-NT can also be fulfilled by simply introducing soluble AgCl in the melts. The present study provides a template-free preparation and an in situ surface modification of Si-NT.

Top-Down Synthesis of Silicon/Carbon Composite Anode Materials for Lithium-Ion Batteries: Mechanical Milling and Etching

Lithium-ion batteries (LIBs) providing high energy and power densities as well as long cycle life are in high demand for various applications. Benefitting from its high theoretical specific charge capacity of ≈4200 mAh g^-1 and natural abundance, Si is nowadays considered as one of the most promising anode candidates for high-energy-density LIBs. However, its huge volume change during cycling prevents its widespread commercialization. Si/C-based electrodes, fabricated through top-down mechanical-milling technique and etching, could be particularly promising since they can adequately accommodate the Si volume expansion, buffer the mechanical stress, and ameliorate the interface/surface stability. In this Review, the current progresses in the top-down synthesis of Si/C anode materials for LIBs from inexpensive Si sources via the combination of low-cost, simple, scalable, and efficient ball-milling and etching processes are summarized. Various Si precursors as well as etching routes are highlighted in this Review. This review would be a guide for fabricating high-performance Si-based anodes.

Saclike-silicon nanoparticles anchored in ZIF-8 derived spongy matrix as high-performance anode for lithium-ion batteries

The carbon layer with good electrical conductivity and outstanding mechanical stability are essential in designing high-performance silicon/carbon (Si/C) anodes to replace the commercial graphite in lithium-ion batteries (LIBs). In terms of solving the two inherent defects of poor conductivity and big volume change of silicon, we fabricate a spongy carbon matrix derived from ZIF-8 to anchor saclike silicon synthesized by molten salt magnesiothermic reduction method. This spongy matrix can anchor saclike silicon to provide a stable reaction interface and support fast electronic transmission. At the same time, buffer space in saclike Si nanoparticles and spongy matrix can synergistically accommodate the volume change of Si to maintain the integrity of the electrode. The resulting composite with a high Si content of 77.58% exhibits good capacities of 1448 mAh g^-1 at 2 A g^-1 and 848 mAh g^-1 at 4 A g^-1 after 500 cycles. High initial coulombic efficiency of 84% at 0.2 A g^-1 is also exhibited in the first three activation cycles. Therefore, this novel multifunctional N-doped spongy matrix can supply multifaceted benefits in accommodation of volumetric variation, enhancement of conductivity, and integrity of structure during cycling.

Engineering Carbon Distribution in Silicon-Based Anodes at Multiple Scales

The successful commercialization of promising silicon-based anode materials has been hampered by their poor cycling stability caused by the huge volume change. Integration of the carbon matrix with silicon-based (C/Si-based) anode materials has been demonstrated to be a powerful solution to achieve satisfactory electrochemical performance. This minireview aims to outline recent developments on C/Si-based composites, with the emphasis on the importance of carbon distribution at multiple scales. In addition, the forms of the carbon framework (carbon sources and doping of heteroatoms) have been summarized. Particularly, a novel C/Si-based hybrid with carbon distributed at the atomic scale has been highlighted.

Coupling of a conductive Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 metal-organic framework with silicon nanoparticles for use in high-capacity lithium-ion batteries

A composite of Si nanoparticles (SiNPs) and a two-dimensional (2D) porous conductive Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) metal-organic framework (MOF), namely Si/Ni3(HITP)2, is suggested as a potential anode material for Li-ion batteries (LIBs). The Ni3(HITP)2 MOF with a carbon backbone and evenly dispersed Ni and N heteroatoms showed high potential for mitigating the volume expansion of Si and enhancing the electronic conductivity as well as Li storage ability of the Si/Ni3(HITP)2 anode. The Si/Ni3(HITP)2 electrode delivered a reversible capacity of 2657 mA h g-1 after 100 cycles of discharge-charge at a rate of 0.1C. Moreover, at a high rate of 1C, the Si/Ni3(HITP)2 electrode maintained a reversible capacity of 876 mA h g-1 even after 1000 cycles. The different rate capacities were 1655, 1129, and 721 mA h g-1 at 5C, 10C and 20C, respectively. The excellent electrochemical performance of the Si/Ni3(HITP)2 electrode in terms of improved cycle life and rate capability results from the open channels of the MOF network, which are beneficial for the movement of Li+ ions through the electrolyte to the electrode and the mitigation of stress by volume expansion of Si. We believe that the coupling of conductive Ni3(HITP)2 with Si is a potential way to make an anode for high-performance LIBs.

A carbon network strategy to synthesize silicon–carbon anodes toward regulated morphologies during molten salt reduction

In this work, a carbon network strategy was proposed to prepare Si/SiO_x/C anodes with regulated morphologies during molten salt reduction.

Preparation and Performance of Si3N4 Hollow Microspheres by the Template Method and Carbothermal Reduction Nitridation

Silicon nitride (Si₃N₄) hollow microspheres with smaller particle size and narrower distribution can be used to prepare closed-cell ceramics as the pore-forming agent to improve heat insulation performance and wave-transparent performance of porous Si₃N₄ ceramics. In this work, Si₃N₄ hollow microspheres with a diameter of about 1 μm and a wall thickness of approximately 150 nm were prepared by using the template method combined with the carbothermal reduction nitridation method. The optimum preparation temperature of the Si₃N₄ hollow microspheres is 1450 °C. The morphology, microstructure, and phase composition of the prepared Si₃N₄ hollow microspheres were characterized. The formation mechanism of the Si₃N₄ hollow microspheres was discussed. The dielectric properties of Si₃N₄ hollow microspheres were measured using the waveguide method at 8.2-12.4 GHz. The results show that the wave-transparent performance of the Si₃N₄ hollow microspheres is similar to those of α-Si₃N₄ particles. It can be used as the pore-forming agent or matrix for preparing lightweight, heat-insulating, wave-transparent, and high-strength porous Si₃N₄ ceramics.

Synthesis of Porous Si/C Composite Nanosheets from Vermiculite with a Hierarchical Structure as a High-Performance Anode for Lithium-Ion Battery

Silicon nanosheets are fascinating anode materials for lithium-ion batteries because of their high specific capacities, structural stability, and fast kinetics in alloying/dealloying with Li. The nanosheets can be synthesized through chemical vapor deposition (CVD), topochemical reaction, and templating method. After coating with a carbon nanolayer, they exhibit enhanced electrochemical performance. However, it is challenging to synthesize ultrathin carbon-coated silicon nanosheets. In this work, porous silicon/carbon (pSi/C) composite nanosheets are synthesized by reducing the carbon-coated expanded vermiculite with metallic Al in the molten salts. The as-prepared pSi/C nanosheets retain the layered nanostructure of vermiculite, with a thickness of less than 50 nm. The carbon nanolayer serves as the diffusion barrier and mechanical support for the growth of mesoporous silicon nanosheets. The anode of pSi/C nanosheets achieves remarkable electrochemical performance, exhibiting a reversible capacity of 1837 mA h g^-1 at 4 A g^-1 and retaining 71.5% of the initial capacity after 500 cycles. The process can be extended to the synthesis of the pSi/C composite nanotube by using other carbon-coated silicate templates such as halloysite.

Surface-Bound Silicon Nanoparticles with a Planar-Oriented N-Type Polymer for Cycle-Stable Li-Ion Battery Anode

Silicon is now well-recognized to be a promising alternative anode for advanced lithium-ion batteries because of its highest capacity available today; however, its insufficiently high Coulombic efficiency upon cycling remains a major challenge for practical application. To overcome this challenge, we have developed a facile mechanochemical method to synthesize a core-shell-structured Si/polyphenylene composite (Si/PPP) with a n-type conductive PPP layer tightly bonded in a planar orientation to the surfaces of Si nanocores. Because of its compactness and flexibility, the outer PPP layer can protect the Si core from contacting the electrolyte and maintaining the structural stability of electrode/electrolyte interface during cycles. As a result, the Si/PPP anode demonstrated a high reversible capacity of ∼2387 mAh g^-1, a stable cycleability with 88.5% capacity retention over 500 cycles, and, particularly, a high Coulombic efficiency of 99.7% upon extended cycling, offering a new insight for future development of high-capacity and cycle-stable Si anode.

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.

In situ synthesis of a silicon flake/nitrogen-doped graphene-like carbon composite from organoclay for high-performance lithium-ion battery anodes

A silicon flake/nitrogen-doped graphene-like carbon composite was prepared from organoclay via an in situ strategy, involving carbonization followed by low-temperature aluminothermic reduction. The pre-formed carbon sheets within the confined interlayer space of clay acted as nanotemplates for in situ synthesizing silicon flakes. As a lithium-ion battery anode, the composite exhibited excellent electrochemical properties.