5L-Scale Magnesio-Milling Reduction of Nanostructured SiO2 for High Capacity Silicon Anodes in Lithium-Ion Batteries

Mechanochemical reduction of clay minerals to porous silicon nanoflakes for high-performance lithium-ion battery anodes

Hierarchically porous silicon nanoflakes were synthesized from natural talc via a mechanochemical reduction method, which showed great potential in the scalable production of silicon nanoflakes due to the abundant precursor and facile strategy. The unique layered structure and chemical composition of talc enabled the formation of two-dimensional nanostructured silicon without any additional templates. As lithium-ion battery anodes, the silicon nanoflakes showed excellent electrochemical properties.

Encapsulation of Silicon Nano Powders via Electrospinning as Lithium Ion Battery Anode Materials

Silicon-containing polyester from tetramethoxysilane, ethylene glycol, and o-Phthalic anhydride were used as encapsulating materials for silicon nano powders (SiNP) via electrospinning, with Polyacrylonitrile (PAN) as spinning additives. In the correct quantities, SiNP could be well encapsulated in nano fibers (200-400 nm) using scanning electron microscopy (SEM). The encapsulating materials were then carbonized to a Si-O-C material at 755 °C (Si@C-SiNF-5 and Si@C-SiNF-10, with different SiNP content). Fiber structure and SiNP crystalline structure were reserved even after high-temperature treatment, as SEM and X-ray diffraction (XRD) verified. When used as lithium ion battery (LIB) anode materials, the cycling stability of SiNPs increased after encapsulation. The capacity of SiNPs decreased to ~10 mAh/g within 30 cycles, while those from Si@C-SiNF-5 and Si@C-SiNF-10 remained over 500 mAh/g at the 30th cycle. We also found that adequate SiNP content is necessary for good encapsulation and better cycling stability. In the anode from Si@C-SiNF-10 in which SiNPs were not well encapsulated, fibers were broken and pulverized as SEM confirmed; thus, its cycling stability is poorer than that from Si@C-SiNF-5.

Modified preparation of Si@C@TiO2 porous microspheres as anodes for high-performance lithium-ion batteries

Microscale porous silicon materials have shown great application potential as anodes for next-generation lithium-ion batteries (LIBs); however, they face significant challenges, including mechanical structure instability, low intrinsic conductivity, and uncontrollable processing. In this study, a modified etching strategy combined with a facile sol-gel method is demonstrated to prepare microscale porous Si microspheres encapsulated by an inner amorphous carbon shell (≈10 nm) and an outer rigid anatase titanium oxide (TiO₂) shell (≈20 nm) (PSi@C@TiO₂), with the intact porous framework and core-shell-shell spherical structure. The interconnected pores can sufficiently accommodate the expansion of the Si core during lithiation. Moreover, the double shells can not only enhance the kinetic behavior of the PSi@C@TiO₂ microspheres, but can act as a compact fence to force the Si core to expand toward the internal pores during lithiation, ensuring the integrity of the porous spherical structure. As a result, the PSi@C@TiO₂ anodes show greatly superior high specific capacity, excellent rate capability, stable solid-electrolyte interphase (SEI) films and steady mechanical structure. It delivers a high reversible capacity of 1004 mA h g^-1 after 250 cycles at 0.5 A g^-1. This study provides a modified method to prepare microscale porous Si anodes with a stable mechanical structure and long cycle life for LIBs.

Engineering Bamboo Leaves Into 3D Macroporous Si@C Composites for Stable Lithium-Ion Battery Anodes

Silicon is considered as the most promising candidate for anodes of next generation lithium-ion batteries owing to its natural abundance and low Li-uptake potential. Building a macroporous structure would alleviate the volume variation and particle fracture of silicon anodes during cycling. However, the common approaches to fabricate macroporous silicon are complex, costly, and high energy-consuming. Herein, bamboo leaves are used as a sustainable and abundant resource to produce macroporous silicon via a scalable magnesiothermic reduction method. The obtained silicon inherits the natural interconnected network from the BLs and the mesopores from the BL-derived silica are engineered into macropores by selective etching after magnesiothermic reduction. These unique structural advantages lead to superior electrochemical performance with efficient electron/ion transport and cycling stability. The macroporous Si@C composite anodes deliver a high capacity of 1,247.7 mAh g⁻¹ after 500 cycles at a current density of 1.0 A g⁻¹ with a remarkable capacity retention of 98.8% and average Coulombic efficiency as high as 99.52% for the same cycle period. Furthermore, the rate capabilities of the Si@C composites are enhanced by conformal carbon coating, which enables the anode to deliver a capacity of 538.2 mAh g⁻¹ at a high current density of 4.0 A g⁻¹ after 1,000 deep cycles. Morphology characterization verifies the structural integrity of the macroporous Si@C composite anodes. This work demonstrated herein provides a simple, economical, and scalable route for the industrial production of macroporous Si anode materials utilizing BLs as a sustainable source for high-performance LIBs.

Silicon nanosheets derived from silicate minerals: controllable synthesis and energy storage application

Silicon plays a crucial part in developing high-performance energy storage materials, owing to a high specific capacity compared to carbon. Moreover, nanoscale silicon is beneficial for reducing the inherent disadvantage of large volume change during repeated lithiation/de-lithiation, while artificial synthesis methods usually involve complex procedures and high costs. On account of the abundant natural reserve and low cost, the manipulation of silicate minerals is a simple and economical approach to prepare silicon nanosheets. In this regard, this mini review introduces different classes of silicate minerals and summarizes some typical molten salt-assisted reduction methods and other valuable methods applied to prepare silicon nanosheets for energy storage. Finally, the challenges and perspectives in this field are also proposed.

Zn2+-Imidazole Coordination Crosslinks for Elastic Polymeric Binders in High-Capacity Silicon Electrodes

Recent research has built a consensus that the binder plays a key role in the performance of high-capacity silicon anodes in lithium-ion batteries. These anodes necessitate the use of a binder to maintain the electrode integrity during the immense volume change of silicon during cycling. Here, Zn2+-imidazole coordination crosslinks that are formed to carboxymethyl cellulose backbones in situ during electrode fabrication are reported. The recoverable nature of Zn2+-imidazole coordination bonds and the flexibility of the poly(ethylene glycol) chains are jointly responsible for the high elasticity of the binder network. The high elasticity tightens interparticle contacts and sustains the electrode integrity, both of which are beneficial for long-term cyclability. These electrodes, with their commercial levels of areal capacities, exhibit superior cycle life in full-cells paired with LiNi0.8Co0.15Al0.05O2 cathodes. The present study underlines the importance of highly reversible metal ion-ligand coordination chemistries for binders intended for high capacity alloying-based electrodes.

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.

Direct Synthesis of Methyl Chlorosilanes from Pd-Mg-SiO2 Substrates Using Mechanochemistry

The direct reaction of methyl chloride with magnesium and palladium infused silica substrates to synthesize methyl chlorosilanes is reported. First, high energy ball milling on solid Mg-SiO₂ mixtures produces elemental silicon and MgO. When PdCl₂ is infused into the mixture, after additional ball milling and high-temperature reduction under H₂ , dipalladium silicide (Pd₂ Si) is produced. The silicon of the Pd₂ Si readily reacts with MeCl under Müller-Rochow reaction conditions, to produce methyl chlorosilanes at yield ratios analogous to those of the traditional process. The dominant product is Me₂ SiCl₂ (selectivity > 30%), followed by MeSiCl₃ and Me₃ SiCl, with minor amounts of the remaining chlorosilanes. Silicon conversion exceeds 20% for most of the substrates. The elemental palladium, which remains within the Pd-Mg-SiO₂ contact mass is re-converted to Pd₂ Si at the next H₂ /high-temperature treatment and reacts again with MeCl to repeat the methyl chlorosilane production. In principle, the resulting cycle of the mechanochemically induced formation of Pd₂ Si followed by the reaction with MeCl can be repeated until the starting SiO₂ converts completely to methyl chlorosilanes.

Swelling-Controlled Double-Layered SiOx/Mg2SiO4/SiOx Composite with Enhanced Initial Coulombic Efficiency for Lithium-Ion Battery

Si-based anode materials are considered as potential materials for high-energy lithium-ion batteries (LIBs) with the advantages of high specific capacities and low operating voltages. However, significant initial capacity loss and large volume variations during cycles are the primary restrictions for the practical application of Si-based anodes. Herein, we propose an affordable and scalable synthesis of double-layered SiO_x/Mg₂SiO₄/SiO_x composites through the magnesiothermic reduction of micro-sized SiO with Mg metal powder at 750 °C for 2 h. The distinctive morphology and microstructure of the double-layered SiO_x/Mg₂SiO₄/SiO_x composite are beneficial as they remarkably improve the reversibility in the first cycle and completely suppress the volume variations during cycling. In our material design, the outermost layer with a highly porous SiO_x structure provides abundant active sites by securing a pathway for efficient access to electrons and electrolytes. The inner layer of Mg₂SiO₄ can constrain the large volume expansion to increase the initial Coulombic efficiency (ICE). Owing to these promising structural features, the composite prepared with a 2:1 molar ratio of SiO to Mg exhibited initial charge and discharge capacities of 1826 and 1381 mA h g^-1, respectively, with an ICE of 75.6%. Moreover, it showed a stable cycle performance, maintaining high capacity retention of up to >86.0% even after 300 cycles. The proposed approach provides practical insight into the mass production of advanced anode materials for high-energy LIBs.

Large Field Enhancement of Nanocoral Structures on Porous Si Synthesized from Rice Husks

Silicon (Si) is a highly abundant, environmentally benign, and durable material and is the most popular semiconductor material; and it is used for the field enhancement of dielectric materials. Porous Si (PSi) exhibits high functionality due to its specific structure. However, the field enhancement of PSi has not been clarified sufficiently. Herein, we present the field enhancement of PSi by the fluorescence intensity enhancement of a dye molecule. The raw material used for producing PSi was rice husk, a biomass material. A nanocoral structure, consisting of spheroidal structures on the surface of PSi, was observed when PSi was subjected to chemical processes and pulsed laser melting, and it demonstrated large field enhancement with an enhancement factor (EF) of up to 545. Confocal microscopy was used for EF mapping of samples before and after laser melting, and the maps were superimposed on nanoscale scanning electron microscope images to highlight the EF effect as a function of microstructure. Nanocoral Si with high EF values were also evaluated by analyzing the porosity from gas adsorption measurements. Nanocoral Si was responsible for the high EF, according to thermodynamic calculations and agreement between experimental and calculation results as determined by Mie scattering theory.

Facile and scalable synthesis of silicon nanowires from waste rice husk silica by the molten salt process

Designing nanostructured silicon, such as in the form of nanoparticles, wires, and porous structures, for high-performance Li-ion electrodes, has progressed significantly. These approaches have largely overcome the capacity fading of silicon electrodes from volume expansion during lithiation/de-lithiation. However, they involve high costs, complex processes, and hazardous precursors. Herein, we propose an electrochemical fabrication of silicon nanowires from waste rice husks via a molten salt process based on electrodeoxidation. The addition of NiO as an electric conductor improved the production efficiency and created pores in the nanowires after washing. The electrically produced high-purity silicon yielded high capacity, and the nanowires provided sufficient free volume to accommodate silicon electrode expansion, resulting in improved cycle life. The converted silicon nanowires from the molten salt process will help develop sustainable energy storage materials.

Hierarchical Carbon-Coated Ball-Milled Silicon: Synthesis and Applications in Free-Standing Electrodes and High-Voltage Full Lithium-Ion Batteries

Lithium-ion batteries have been regarded as one of the most promising energy storage devices, and development of low-cost batteries with high energy density is highly desired so that the cost per watt-hour ($/Wh) can be minimized. In this work, we report using ball-milled low-cost silicon (Si) as the starting material and subsequent carbon coating to produce low-cost hierarchical carbon-coated (HCC) Si. The obtained particles prepared from different Si sources all show excellent cycling performance of over 1000 mAh/g after 1000 cycles. Interestingly, we observed in situ formation of porous Si, and it is well confined in the carbon shell based on postcycling characterization of the hierarchical carbon-coated metallurgical Si (HCC-M-Si) particles. In addition, lightweight and free-standing electrodes consisting of the HCC-M-Si particles and carbon nanofibers were fabricated, which achieved 1015 mAh/g after 100 cycles based on the total mass of the electrodes. Compared with conventional electrodes, the lightweight and free-standing electrodes significantly improve the energy density by 745%. Furthermore, LiCoO₂ and LiNi_0.5Mn_1.5O₄ cathodes were used to pair up with the HCC-M-Si anode to fabricate full cells. With LiNi_0.5Mn_1.5O₄ as cathode, an energy density up to 547 Wh/kg was achieved by the high-voltage full cell. After 100 cycles, the full cell with a LiNi_0.5Mn_1.5O₄ cathode delivers 46% more energy density than that of the full cell with a LiCoO₂ cathode. The systematic investigation on low-cost Si anodes together with their applications in lightweight free-standing electrodes and high-voltage full cells will shed light on the development of high-energy Si-based lithium-ion batteries for real applications.

Green, Scalable, and Controllable Fabrication of Nanoporous Silicon from Commercial Alloy Precursors for High-Energy Lithium-Ion Batteries

Silicon is considered as one of the most favorable anode materials for next-generation lithium-ion batteries. Nanoporous silicon is synthesized via a green, facile, and controllable vacuum distillation method from the commercial Mg₂Si alloy. Nanoporous silicon is formed by the evaporation of low boiling point Mg. In this method, the magnesium metal from the Mg₂Si alloy can be recycled. The pore sizes of nanoporous silicon can be secured by adjusting the distillated temperature and time. The optimized nanoporous silicon (800 °C, 0.5 h) delivers a discharge capacity of 2034 mA h g^-1 at 200 mA g^-1 for 100 cycles, a cycling stability with more than 1180 mA h g^-1 even after 400 cycles at 1000 mA g^-1, and a rate capability of 855 mA h g^-1 at 5000 mA g^-1. The electrochemical properties might be ascribed to its porous structure, which may accommodate large volume change during the cycling process. These results suggest that the green, scalable, and controllable approach may offer a pathway for the commercialization of high-performance Si anodes. This method may also be extended to construct other nanoporous materials.

Metallothermic Reduction of Silica Nanoparticles to Porous Silicon for Drug Delivery Using New and Existing Reductants

In this study, the influence of metals (Mg, Al, and Ca) and reaction conditions (time, temperature, and metal grain size) on the metallothermic reduction of Stöber silica nanoparticles (NPs) to form porous Si has been explored. Mg metal was found to be an effective reducing agent even at temperatures below its melting point; however, it also induced a high degree of structural damage and morphology change. Al was effective in reducing silica NPs only at its melting point or above, but the resulting particles retained a higher degree of structural morphology as compared to those reduced using Mg. Ca was found to be ineffective in reducing silica. A new reductant, a mixture of 70 % Mg and 30 % Al, was found to induce the least amount of morphology change, and the reactions proceeded at a temperature (450 °C) lower than those required with Mg or Al individually. Furthermore, porous Si NPs obtained using Mg, Al, and the mixture of 70 % Mg and 30 % Al as reductants have been investigated as carriers for ibuprofen loading and release. Porous Si obtained from reductions with Mg and the Mg/Al mixture showed higher drug loading and a sustained drug release profile, whereas porous Si obtained from Al reduction had lower loading and showed a conventional release profile over 24 h.

The emerging era of supramolecular polymeric binders in silicon anodes

Silicon (Si) anode is among the most promising candidates for the next-generation high-capacity electrodes in Li-ion batteries owing to its unparalleled theoretical capacity (4200 mA h g^-1 for Li_4.4Si) that is approximately 10 times higher than that of commercialized graphitic anodes (372 mA h g^-1 for LiC₆). The battery community has witnessed substantial advances in research on new polymeric binders for silicon anodes mainly due to the shortcomings of conventional binders such as polyvinylidene difluoride (PVDF) to address problems caused by the massive volume change of Si (300%) upon (de)lithiation. Unlike conventional battery electrodes, polymeric binders have been shown to play an active role in silicon anodes to alleviate various capacity decay pathways. While the initial focus in binder research was primarily to maintain the electrode morphology, it has been recently shown that polymeric binders can in fact help to stabilize cracked Si microparticles along with the solid-electrolyte-interphase (SEI) layer, thus substantially improving the electrochemical performance. In this review article, we aim to provide an in-depth analysis and molecular-level design principles of polymeric binders for silicon anodes in terms of their chemical structure, superstructure, and supramolecular interactions to achieve good electrochemical performance. We further highlight that supramolecular chemistry offers practical tools to address challenging problems associated with emerging electrode materials in rechargeable batteries.

Facile Synthesis of Si@SiC Composite as an Anode Material for Lithium-Ion Batteries

Here, we propose a simple method for direct synthesis of a Si@SiC composite derived from a SiO₂@C precursor via a Mg thermal reduction method as an anode material for Li-ion batteries. Owing to the extremely high exothermic reaction between SiO₂ and Mg, along with the presence of carbon, SiC can be spontaneously produced with the formation of Si. The synthesized Si@SiC was composed of well-mixed SiC and Si nanocrystallites. The SiC content of the Si@SiC was adjusted by tuning the carbon content of the precursor. Among the resultant Si@SiC materials, the Si@SiC-0.5 sample, which was produced from a precursor containing 4.37 wt % of carbon, exhibits excellent electrochemical characteristics, such as a high first discharge capacity of 1642 mAh g^-1 and 53.9% capacity retention following 200 cycles at a rate of 0.1C. Even at a high rate of 10C, a high reversible capacity of 454 mAh g^-1 was obtained. Surprisingly, at a fixed discharge rate of C/20, the Si@SiC-0.5 electrode delivered a high capacity of 989 mAh g^-1 at a charge rate of 20C. In addition, a full cell fabricated by coupling a lithiated Si@SiC-0.5 anode and a LiCoO₂ cathode exhibits excellent cyclability over 50 cycles. This outstanding electrochemical performance of Si@SiC-0.5 is attributed to the SiC phase, which acts as a buffer layer that stabilizes the nanostructure of the Si active phase and enhances the electrical conductivity of the electrode.

Rice husk-originating silicon-graphite composites for advanced lithium ion battery anodes

Rice husk is produced in a massive amount worldwide as a byproduct of rice cultivation. Rice husk contains approximately 20 wt% of mesoporous SiO₂. We produce mesoporous silicon (Si) by reducing the rice husk-originating SiO₂ using a magnesio-milling process. Taking advantage of meso-porosity and large available quantity, we apply rice husk-originating Si to lithium ion battery anodes in a composite form with commercial graphite. By varying the mass ratio between these two components, trade-off relation between specific capacity and cycle life was observed. A controllable pre-lithiation scheme was adopted to increase the initial Coulombic efficiency and energy density. The series of electrochemical results suggest that rice husk-originating Si-graphite composites are promising candidates for high capacity lithium ion battery anodes, with the prominent advantages in battery performance and scalability.

Delicate Structural Control of Si-SiOx-C Composite via High-Speed Spray Pyrolysis for Li-Ion Battery Anodes

Despite the high theoretical capacity, silicon (Si) anodes in lithium-ion batteries have difficulty in meeting the commercial standards in various aspects. In particular, the huge volume change of Si makes it very challenging to simultaneously achieve high initial Coulombic efficiency (ICE) and long-term cycle life. Herein, we report spray pyrolysis to prepare Si-SiO_x composite using an aqueous precursor solution containing Si nanoparticles, citric acid, and sodium hydroxide (NaOH). In the precursor solution, Si nanoparticles are etched by NaOH with the production of [SiO₄]^4-. During the dynamic course of spray pyrolysis, [SiO₄]^4- transforms to SiO_x matrix and citric acid decomposes to carbon surface layer with the assistance of NaOH that serves as a decomposition catalyst. As a result, a Si-SiO_x composite, in which Si nanodomains are homogeneously embedded in the SiO_x matrix with carbon surface layer, is generated by a one-pot process with a residence time of only 3.5 s in a flow reactor. The optimal composite structure in terms of Si domain size and Si-to-O ratio exhibited excellent electrochemical performance, such as reversible capacity of 1561.9 mAh g^-1 at 0.06C rate and ICE of 80.2% and 87.9% capacity retention after 100 cycles at 1C rate.