Review of porous silicon preparation and its application for lithium-ion battery anodes

Dendritic Copper Current Collectors as a Capacity Boosting Material for Polymer-Templated Si/Ge/C Anodes in Li-Ion Batteries

Dendritic copper offers a highly effective method for synthesizing porous copper anodes due to its intricate branching structure. This morphology results in an elevated surface area-to-volume ratio, facilitating shortened electron pathways during aqueous and electrolyte permeation. Here, we demonstrate a procedure for a time- and cost-efficient synthesis routine of fern-like copper microstructures as a host for polymer-templated Si/Ge/C thin films. Dissolvable Zintl clusters and sol-gel chemistry are used to synthesize nanoporous coating as the anode. Cyclic voltammetry (CV) with KOH as the electrolyte is used to estimate the surface area increase in the dendritic copper current collectors (CCs). Half cells are assembled and tested with battery-related techniques such as CV, galvanostatic cycling, and electrochemical impedance spectroscopy, showing a capacity increase in the dendritic copper cells. Energy-dispersive X-ray spectroscopy is used to estimate the removal of K in the bulk after oxidizing the Zintl phase K12Si8Ge9 in the polymer/precursor blend with SiCl4. Furthermore, scanning electron microscopy images are provided to depict the thin films after synthesis and track the degradation of the half cells after cycling, revealing that the morphological degradation through alloying/dealloying is reduced for the dendritic Cu CC anodes as compared with the bare reference. Finally, we highlight this time- and cost-efficient routine for synthesizing this capacity-boosting material for low-mobility and high-capacity anode coatings.

Lithiated porous silicon nanowires stimulate periodontal regeneration

Periodontal disease is a significant burden for oral health, causing progressive and irreversible damage to the support structure of the tooth. This complex structure, the periodontium, is composed of interconnected soft and mineralised tissues, posing a challenge for regenerative approaches. Materials combining silicon and lithium are widely studied in periodontal regeneration, as they stimulate bone repair via silicic acid release while providing regenerative stimuli through lithium activation of the Wnt/β-catenin pathway. Yet, existing materials for combined lithium and silicon release have limited control over ion release amounts and kinetics. Porous silicon can provide controlled silicic acid release, inducing osteogenesis to support bone regeneration. Prelithiation, a strategy developed for battery technology, can introduce large, controllable amounts of lithium within porous silicon, but yields a highly reactive material, unsuitable for biomedicine. This work debuts a strategy to lithiate porous silicon nanowires (LipSiNs) which generates a biocompatible and bioresorbable material. LipSiNs incorporate lithium to between 1% and 40% of silicon content, releasing lithium and silicic acid in a tailorable fashion from days to weeks. LipSiNs combine osteogenic, cementogenic and Wnt/β-catenin stimuli to regenerate bone, cementum and periodontal ligament fibres in a murine periodontal defect.

Growth of Vertical Graphene Sheets on Silicon Nanoparticles Well-Dispersed on Graphite Particles for High-Performance Lithium-Ion Battery Anode

With rapidly increasing demand for high energy density, silicon (Si) is greatly expected to play an important role as the anode material of lithium-ion batteries (LIBs) due to its high specific capacity. However, large volume expansion for silicon during the charging process is still a serious problem influencing its cycling stability. Here, a Si/C composite of vertical graphene sheets/silicon/carbon/graphite (VGSs@Si/C/G) is reported to address the electrochemical stability issues of Si/graphite anodes, which is prepared by adhering Si nanoparticles on graphite particles with chitosan and then in situ growing VGSs by thermal chemical vapor deposition. As a promising anode material, due to the buffering effect of VGSs and tight bonding between Si and graphite particles, the composite delivers a high reversible capacity of 782.2 mAh g-1 after 1000 cycles with an initial coulombic efficiency of 87.2%. Furthermore, the VGSs@Si/C/G shows a diffusion coefficient of two orders higher than that without growing the VGSs. The full battery using VGSs@Si/C/G anode and LiNi0.8 Co0.1 Mn0.1 O2 cathode achieves a high gravimetric energy density of 343.6 Wh kg-1 , a high capacity retention of 91.5% after 500 cycles and an excellent average CE of 99.9%.

Influence of bridge structure manipulation on the electrochemical performance of π-conjugated molecule-bridged silicon quantum dot nanocomposite anode materials for lithium-ion batteries

To assess the influence of bridge structure manipulation on the electrochemical performance of π-conjugated molecule-bridged silicon quantum dot (Si QD) nanocomposite (SQNC) anode materials, we prepared two types of SQNCs by Sonogashira cross-coupling and hydrosilylation reactions; one is SQNC-VPEPV, wherein the Si QDs are covalently bonded by vinylene (V)-phenylene (P)-ethynylene (E)-phenylene-vinylene, and the other is SQNC-VPV. By comparing the electrochemical performances of the SQNCs, including that of the previously reported SQNC-VPEPEPV, we found that the SQNC with the highest specific capacity varied depending on the applied current density; SQNC-VPEPV (1420 mA h g^-1) > SQNC-VPV (779 mA h g^-1) > SQNC-VPEPEPV(465 mA h g^-1) at 800 mA g^-1, and SQNC-VPV (529 mA h g^-1) > SQNC-VPEPEPV (53 mA h g^-1) > SQNC-VPEPV (7 mA h g^-1) at 2000 mA g^-1. To understand this result, we performed EIS and GITT measurements of the SQNCs. In the course of investigating the lithium-ion diffusion coefficient, charge/discharge kinetics, and electrochemical performance of the SQNC anode materials, we found that electronic conductivity is a key parameter for determining the electrochemical performance of the SQNC. Two probable causes for the unique behavior of the electrochemical performances of the SQNCs are anticipated: (i) the SQNC with predominant electronic conductivity is varied depending on the current density applied during the cell operation, and (ii) the degree of surface oxidation of the Si QDs in the SQNCs varies depending on the structures of the surface organic molecules of the Si QDs and the bridging molecules of the SQNCs. Therefore, differences in the amount of oxides (SiO₂)/suboxides (SiO_x) on the surface of Si QDs lead to significant differences in conductivity and electrochemical performance between the SQNCs.

Hydrogen Bond-Enabled High-ICE Anode for Lithium-Ion Battery Using Carbonized Citric Acid-Coated Silicon Flake in PAA Binder

A silicon-based lithium-ion battery (LIB) anode is extensively studied because of silicon's abundance, high theoretical specific capacity (4200 mAh/g), and low operating potential versus lithium. Technical barriers to large-scale commercial applications include the low electrical conductivity and up to about 400% volume changes of silicon due to alloying with lithium. Maintaining the physical integrity of individual silicon particles and the anode structure is the top priority. We use strong hydrogen bonds between citric acid (CA) and silicon to firmly coat CA on silicon. Carbonized CA (CCA) enhances electrical conductivity of silicon. Polyacrylic acid (PAA) binder encapsulates silicon flakes by strong bonds formed by abundant COOH functional groups in PAA and on CCA. It results in excellent physical integrity of individual silicon particles and the whole anode. The silicon-based anode shows high initial coulombic efficiency, around 90%, and the capacity retention of 1479 mAh/g after 200 discharge-charge cycles at 1 A/g current. At 4 A/g, the capacity retention of 1053 mAh/g was achieved. A durable high-ICE silicon-based LIB anode capable of high discharge-charge current has been reported.

High-performance SiGe anode materials obtained by dealloying a Sr-modified Al-Si-Ge eutectic precursor

In exploring the anode materials for high efficiency Li-ion batteries, it has been found that the electrochemical performance of Si can be enhanced via alloying with Ge. In the present work, we modified the Al-Si-Ge eutectic ribbons as the precursor by adding a trace of Sr to the alloy. The SiGe particles obtained by dealloying the Al-Si-Ge eutectic precursor have a porous coral-like nano-architecture with numerous fibrous branches towards various directions. Because of the large surface area and porosity, the as-prepared Sr-modified SiGe anode delivers an excellent capacity of 1166.6 mA h g^-1 at 0.1 A g^-1 after 100 cycles with a fantastic initial coulombic efficiency of 83.62%. Besides, it has a superior rate performance with a reversible capacity of 675.3 mA h g^-1 at the current density of 8 A g^-1. It is demonstrated that the modification treatment that is widely used in metallurgy is also a promising strategy to synthesize high-performance battery electrodes and other energy storage materials.

Thermal Properties of Porous Silicon Nanomaterials

The thermal properties, including the heat capacity, thermal conductivity, effusivity, diffusivity, and phonon density of states of silicon-based nanomaterials are analyzed using a molecular dynamics calculation. These quantities are calculated in more detail for bulk silicon, porous silicon, and a silicon aerocrystal (aerogel), including the passivation of the porous internal surfaces with hydrogen, hydroxide, and oxygen ions. It is found that the heat capacity of these materials increases monotonically by up to 30% with an increase in the area of the porous inner surface and upon its passivation with these ions. This phenomenon is explained by a shift of the phonon density of states of the materials under study to the low-frequency region. In addition, it is shown that the thermal conductivity of the investigated materials depends on the degree of their porosity and can be changed significantly upon the passivation of their inner surface with different ions. It is demonstrated that, in the various simulated types of porous silicon, the thermal conductivity changes by 1-2 orders of magnitude compared with the value for bulk silicon. At the same time, it is found that the nature of the passivation of the internal nanosilicon surfaces affects the thermal conductivity. For example, the passivation of the surfaces with hydrogen does not significantly change this parameter, whereas a passivation with oxygen ions reduces it by a factor of two on average, and passivation with hydroxyl ions increases the thermal conductivity by a factor of 2-3. Similar trends are observed for the thermal effusivities and diffusivities of all the types of nanoporous silicon under passivation, but, in that case, the changes are weaker (by a factor of 1.5-2). The ways of tuning the thermal properties of the new nanostructured materials are outlined, which is important for their application.

High-performance Bloch surface wave biosensor based on a prism-coupled porous silicon composite structure for the detection of hemoglobin

Biosensors have various potential applications in biomedical research and clinical diagnostic, especially in detection of biomolecules in highly diluted solutions. In this study, a high-performance Bloch surface wave biosensor was constructed for the detection of hemoglobin. The procedure consisted of designing a porous silicon-based Kretschmann configuration to ensure excitation of the Bloch surface wave. The performance of the resulting sensor was then optimized by adjusting the buffer layer parameters based on the impedance matching method. The results showed an increase in the quality factor and figure of merit of the biosensor as a function of the decrease in thickness and refractive index of the buffer layer. The combination of the two optimization methods resulted in the quality factor and figure of merit of the optimized biosensor reaching as high as Q = 6967.4 and FOM = 11050RIU^-1, respectively. In sum, the designed biosensor with high performance looks promising for future detection of hemoglobin.

Synergetic Effect of Hybrid Conductive Additives for High-Capacity and Excellent Cyclability in Si Anodes

Silicon is a promising anode material that can increase the theoretical capacity of lithium-ion batteries (LIBs). However, the volume expansion of silicon remains a challenge. In this study, we employed a novel combination of conductive additives to effectively suppress the volume expansion of Si during charging/discharging cycles. Rather than carbon black (CB), which is commonly used in SiO anodes, we introduced single-walled carbon nanotubes (SWCNTs) as a conductive additive. Owing to their high aspect ratio, CNTs enable effective connection of SiO particles, leading to stable electrochemical operation to prevent volume expansion. In addition, we explored a combination of CB and SWCNTs, with results showing a synergetic effect compared to a single-component of SWCNTs, as small-sized CB particles can enhance the interface contact between the conductive additive and SiO particles, whereas SWCNTs have limited contact points. With this hybrid conductive additive, we achieved a stable operation of full-cell LIBs for more than 200 cycles, with a retention rate of 91.1%, whereas conventional CB showed a 74.0% specific capacity retention rate.

Ball-Milled Silicon with Amorphous Al2O3/C Hybrid Coating Embedded in Graphene/Graphite Nanosheets with a Boosted Lithium Storage Capability

Electrochemical active silicon has attracted great attention as anodes for lithium-ion batteries owing to a high theoretical capacity of 4200 mA h g^-1. In this work, ball-milled silicon particles with submicron size were strategically modified with a hybrid coating of amorphous alumina and carbon, which simultaneously embedded in a porous framework of in situ exfoliated graphene/graphite nanosheets (GGN). The composite exhibits an enhanced electrochemical performance, including high cycling stability and superior rate capability. An initial discharge capacity of 1294 mA h g^-1 and a reversible charge capacity of 1044 mA h g^-1 at 0.2 A g^-1 can be achieved with a high initial Coulombic efficiency of up to ca. 81%. Additionally, the composite can remain 902 mA h g^-1 after 100 discharge/charge cycles, accounting for a high retention of about 86%. This silicon composite is a promising anode material for high performance lithium-ion batteries with a high energy density, and the facile one-pot fabrication route is low cost and scalable, with a great prospect for practical application.

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.

Fabrication of amorphous hollow mesoporous Si@SiOx nanoboxes as an anode material for enhanced lithium storage performance

Hollow mesoporous Si@SiOx nanoboxes are synthesized successfully by a simple sol–gel reaction of triethoxysilane using Fe2O3 nanocubes as the template, followed by a thermal reduction process and subsequent acid treatment process.

Vertically Aligned n-Type Silicon Nanowire Array as a Free-Standing Anode for Lithium-Ion Batteries

Due to its high theoretical specific capacity, a silicon anode is one of the candidates for realizing high energy density lithium-ion batteries (LIBs). However, problems related to bulk silicon (e.g., low intrinsic conductivity and massive volume expansion) limit the performance of silicon anodes. In this work, to improve the performance of silicon anodes, a vertically aligned n-type silicon nanowire array (n-SiNW) was fabricated using a well-controlled, top-down nano-machining technique by combining photolithography and inductively coupled plasma reactive ion etching (ICP-RIE) at a cryogenic temperature. The array of nanowires ~1 µm in diameter and with the aspect ratio of ~10 was successfully prepared from commercial n-type silicon wafer. The half-cell LIB with free-standing n-SiNW electrode exhibited an initial Coulombic efficiency of 91.1%, which was higher than the battery with a blank n-silicon wafer electrode (i.e., 67.5%). Upon 100 cycles of stability testing at 0.06 mA cm⁻², the battery with the n-SiNW electrode retained 85.9% of its 0.50 mAh cm⁻² capacity after the pre-lithiation step, whereas its counterpart, the blank n-silicon wafer electrode, only maintained 61.4% of 0.21 mAh cm⁻² capacity. Furthermore, 76.7% capacity retention can be obtained at a current density of 0.2 mA cm⁻², showing the potential of n-SiNW anodes for high current density applications. This work presents an alternative method for facile, high precision, and high throughput patterning on a wafer-scale to obtain a high aspect ratio n-SiNW, and its application in LIBs.

Fabrication of Black Silicon via Metal-Assisted Chemical Etching—A Review

The metal-assisted chemical etching (MACE) technique is commonly employed for texturing the wafer surfaces when fabricating black silicon (BSi) solar cells and is considered to be a potential technique to improve the efficiency of traditional Si-based solar cells. This article aims to review the MACE technique along with its mechanism for Ag-, Cu- and Ni-assisted etching. Primarily, several essential aspects of the fabrication of BSi are discussed, including chemical reaction, etching direction, mass transfer, and the overall etching process of the MACE method. Thereafter, three metal catalysts (Ag, Cu, and Ni) are critically analyzed to identify their roles in producing cost-effective and sustainable BSi solar cells with higher quality and efficiency. The conducted study revealed that Ag-etched BSi wafers are more suitable for the growth of higher quality and efficiency Si solar cells compared to Cu- and Ni-etched BSi wafers. However, both Cu and Ni seem to be more cost-effective and more appropriate for the mass production of BSi solar cells than Ag-etched wafers. Meanwhile, the Ni-assisted chemical etching process takes a longer time than Cu but the Ni-etched BSi solar cells possess enhanced light absorption capacity and lower activity in terms of the dissolution and oxidation process than Cu-etched BSi solar cells.

Advanced and Emerging Negative Electrodes for Li-Ion Capacitors: Pragmatism vs. Performance

Li-ion capacitors (LICs) are designed to achieve high power and energy densities using a carbon-based material as a positive electrode coupled with a negative electrode often adopted from Li-ion batteries. However, such adoption cannot be direct and requires additional materials optimization. Furthermore, for the desired device’s performance, a proper design of the electrodes is necessary to balance the different charge storage mechanisms. The negative electrode with an intercalation or alloying active material must provide the high rate performance and long-term cycling ability necessary for LIC functionality—a primary challenge for the design of these energy-storage devices. In addition, the search for new active materials must also consider the need for environmentally friendly chemistry and the sustainable availability of key elements. With these factors in mind, this review evaluates advanced and emerging materials used as high-rate anodes in LICs from the perspective of their practical implementation.

Li8 Si8 , Li10 Si9 , and Li12 Si10 : Assemblies of Lithium-Silicon Aromatic Units

We report the global minima structures of Li₈ Si₈ , Li₁₀ Si₉ , and Li₁₂ Si₁₀ systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters T_d -Li₄ Si₄ and C_2v -Li₆ Si₅ . Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit's aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks.

Recycling Si waste cut from diamond wire into high performance porous Si@SiO2@C anodes for Li-ion battery

The Si particle waste cut from diamond wire in photovoltaic industry is chose as an environmental friendly and low-cost resource for Li-ion battery. In this study, the pollutions of SiO₂ layer, adhered trace metal and organic impurities on the Si particle waste can be removed by the facile processes of corrosion and pyrolysis. The removal ratios of organic and metal impurities were 70.42% and 66.76%, respectively. The different kinetic models for the removal of metal impurities demonstrate that the leaching is more suitable for controlling by second-order reaction of homogeneous models (R² =0.992, m=2). The preparation analysis of porous Si@SiO₂ with a 3D cluster nanoporous structure using a special bubble corrosion method was firstly proposed and discussed intensively. The first discharge and charge capacities of porous Si@SiO₂ @C composites reached 2579.8 and 2184.1 mAh/g, the initial CE reached 84.66%, and the corresponding capacities after 100 cycles reached 1051.4 and 1038.2 mAh/g, which showed the better electrical performance. This study establishes a theoretical basis for recycling Si particle waste cut from diamond wire, and provides technical support for the energy sustainable development.

Effects of SiC and Resorcinol-Formaldehyde (RF) Carbon Coatings on Silicon-Flake-Based Anode of Lithium Ion Battery

Silicon flakes of about 100 × 1000 × 1000 nm in sizes recycled from wastes of silicon wafer manufacturing processes were coated with combined silicon carbide (SiC) and graphitic (Resorcinol-Formaldehyde (RF)) carbon coatings to serve as active materials of the anode of lithium ion battery (LIB). Thermal carbonization of silicon at 1000 °C for 5 h forms 5-nm SiC encapsulating silicon flakes. SiC provides physical strength to help silicon flakes maintain physical integrity and isolating silicon from irreversible reactions with the electrolyte. Lithium diffuses through SiC before alloying with silicon. The SiC buffer layer results in uniform alloying reactions between lithium and silicon on the surface around a silicon flake. RF carbon coatings provide enhanced electrical conductivity of SiC encapsulated silicon flakes. We characterized the coatings and anode by SEM, TEM, FTIR, XRD, cyclic voltammetry (CV), electrochemical impedance spectra (EIS), and electrical resistance measurements. Coin half-cells with combined SiC and RF carbon coatings exhibit an initial Coulombic efficiency (ICE) of 76% and retains a specific capacity of 955 mAh/g at 100th cycle and 850 mAh/g at 150th cycle of repetitive discharge and charge operation. Pre-lithiation of the anode increases the ICE to 97%. The SiC buffer layer reduces local stresses caused by non-uniform volume changes and improves the capacity retention and the cycling life.

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.

Silicon-Based Anode of Lithium Ion Battery Made of Nano Silicon Flakes Partially Encapsulated by Silicon Dioxide

Ubiquitous mobile electronic devices and rapidly increasing electric vehicles demand a better lithium ion battery (LIB) with a more durable and higher specific charge storage capacity than traditional graphite-based ones. Silicon is among the most promising active media since it exhibits ten times of a specific capacity. However, alloying with lithium by silicon and dissociation of the silicon-lithium alloys induce high volume changes and result in pulverization. The loss of electrical contacts by silicon with the current collector of the anode causes rapid capacity decay. We report improved anode cycling performance made of silicon flakes partially encapsulated by silicon dioxide and coated with conductive nanocarbon films and CNTs. The silicon dioxide surface layer on a silicon flake improves the physical integrity for a silicon-based anode. The exposed silicon surface provides a fast transport of lithium ions and electrons. CNTs and nanocarbon films provide electrical connections between silicon flakes and the current collector. We report a novel way of manufacturing silicon flakes partially covered by silicon dioxide through breaking oxidized silicon flakes into smaller pieces. Additionally, we demonstrate an improved cycling life and capacity retention compared to pristine silicon flakes and silicon flakes fully encapsulated by silicon dioxide. Nanocarbon coatings provide conduction channels and further improve the anode performance.

Morphology engineering of silicon nanoparticles for better performance in Li-ion battery anodes

Amorphous silicon nanoparticles were synthesized through pyrolysis of silane gas at temperatures ranging from 575 to 675 °C. According to the used temperature and silane concentration, two distinct types of particles can be obtained: at 625 °C, spherical particles with smooth surface and a low degree of aggregation, but at a higher temperature (650 °C) and lower silane concentration, particles with extremely rough surfaces and high degree of aggregation are found. This demonstrates the importance of the synthesis temperature on the morphology of silicon particles. The two types of silicon nanoparticles were subsequently used as active materials in a lithium half cell configuration, using LiPF₆ in an alkylcarbonate-based electrolyte, in order to investigate the impact of the particles morphology on the cycling performances of silicon anode material. The difference in morphology of the particles resulted in different volume expansions, which impacts the solid electrolyte interface (SEI) formation and, as a consequence, the lifetime of the electrode. Half-cells fabricated from spherical particles demonstrated almost 70% capacity retention for over 300 cycles, while the cells made from the rough, aggregated particles showed a sharp decrease in capacity after the 20^th cycle. The cycling results underline the importance of Si particle engineering and its influence on the lifetime of Si-based materials.

Porous silicon nanomaterials: recent advances in surface engineering for controlled drug-delivery applications

Porous silicon (pSi) nanomaterials are increasingly attractive for biomedical applications due to their promising properties such as simple and feasible fabrication procedures, tunable morphology, versatile surface modification routes, biocompatibility and biodegradability. This review focuses on recent advances in surface modification of pSi for controlled drug delivery applications. A range of functionalization strategies and fabrication methods for pSi-polymer hybrids are summarized. Surface engineering solutions such as stimuli-responsive polymer grafting, stealth coatings and active targeting modifications are highlighted as examples to demonstrate what can be achieved. Finally, the current status of engineered pSi nanomaterials for in vivo applications is reviewed and future prospects and challenges in drug-delivery applications are discussed.

An Amorphous/Crystalline Incorporated Si/SiOx Anode Material Derived from Biomass Corn Leaves for Lithium-Ion Batteries

The fabrication of silicon (Si) anode materials derived from high silica-containing plants enables effective utilization of subsidiary agricultural products. However, the electrochemical performances of synthesized Si materials still require improvement and thus need further structural design and morphology modifications, which inevitably increase preparation time and economic cost. Here, the conversion of corn leaves into Si anode materials is reported via a simple aluminothermic reduction reaction without other modifications. The obtained Si material inherits the structural characteristics of the natural corn leaf template and has many inherent advantages, such as high porosity, amorphous/crystalline mixture structure, and high-valence SiO_x residuals, which significantly enhance the material's structural stability and electrode adhesive strength, resulting in superior electrochemical performances. Rate capability tests show that the material delivers a high capacity of 1200 mA h g^-1 at 8 A g^-1 current density. After 300 cycles at 0.5 A g^-1 , the material maintains a high specific capacity of 2100 mA h g^-1 , with nearly 100% capacity retention during long-term cycling. This study provides an economical route for the industrial production of Si anode materials for Lithium-Ion batteries.

Facile fabrication of sponge-like porous micropillar arrays via an electrochemical process

A large variety of synthetic methods have been developed for hierarchically porous materials by which the performance of a wide range of applications can be dramatically enhanced. Herein, hierarchically porous micropillar arrays are demonstrated by employing electrochemical etching to silicon micropillars. The approach relies on the steering of current flow through the three-dimensional silicon-electrolyte interface to enable nanopores to grow on the entire surface of the micropillars, simultaneously. The pores grow perpendicular to the surface of the micropillars, whereas the pore diameter and porosity vary depending on the locations of the surfaces. The finite element analysis shows that the spatial variation of the pore diameter and porosity is determined by the distribution of current density. Further, the thickness of the porous layer can be tuned by etching time so that sponge-like porous structures are conveniently obtained by regulating the etching time. In addition to the effect of current density flowing through the etched surfaces, the growth of pores also depends on the crystal orientations of the etched surfaces. The etching results on square micropillar arrays and microgroove arrays show that the growth direction and rate of nanopores inside the microstructure also depend on the exposed crystal planes. The facile characteristics of the fabrication method can serve as an effective route for a wide range of applications of porous materials with enhanced capabilities.

Recent development and applications of nanomaterials for cancer immunotherapy

Immunotherapy, which utilizes the patient’s own immune system to fight against cancer, further results in durable antitumor responses and reduces metastasis and recurrence, has become one of the most effective and important cancer therapies along with surgery, radiotherapy, and chemotherapy. Nanomaterials with the advantages of large specific surface, delivery function, and controllable surface chemistry are used to deliver antigens or adjuvants, or both, help to boost immune responses with the imaging function or just act as adjuvants themselves and modulate tumor microenvironment (TME). In this review, recent development and applications of nanomaterials for cancer immunotherapy including delivery systems based on nanomaterials, uniting imaging, self-adjuvants, targeting functions, artificial antigen presenting cells, and TME modulation are focused and discussed.

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.

In Situ Formation of Nanoporous Silicon on a Silicon Wafer via the Magnesiothermic Reduction Reaction (MRR) of Diatomaceous Earth

Successful direct route production of silicon nanostructures from diatomaceous earth (DE) on a single crystalline silicon wafer via the magnesiothermic reduction reaction is reported. The formed porous coating of 6 µm overall thickness contains silicon as the majority phase along with minor traces of Mg, as evident from SEM-EDS and the Focused Ion Beam (FIB) analysis. Raman peaks of silicon at 519 cm^-1 and 925 cm^-1 were found in both the film and wafer substrate, and significant intensity variation was observed, consistent with the SEM observation of the directly formed silicon nanoflake layer. Microstructural analysis of the flakes reveals the presence of pores and cavities partially retained from the precursor diatomite powder. A considerable reduction in surface reflectivity was observed for the silicon nanoflakes, from 45% for silicon wafer to below 15%. The results open possibilities for producing nanostructured silicon with a vast range of functionalities.

Porous Si Partially Filled with Water Molecules-Crystal Structure, Energy Bands and Optical Properties from First Principles

The paper reports the results on first-principles investigation of energy band spectrum and optical properties of bulk and nanoporous silicon. We present the evolution of energy band-gap, refractive indices and extinction coefficients going from the bulk Si of cubic symmetry to porous Si with periodically ordered square-shaped pores of 7.34, 11.26 and 15.40 Å width. We consider two natural processes observed in practice, the hydroxylation of Si pores (introduction of OH groups into pores) and the penetration of water molecules into Si pores, as well as their impact on the electronic spectrum and optical properties of Si superstructures. The penetration of OH groups into the pores of the smallest 7.34 Å width causes a disintegration of hydroxyl groups and forms non-bonded protons which might be a reason for proton conductivity of porous Si. The porosity of silicon increases the extinction coefficient, k, in the visible range of the spectrum. The water structuring in pores of various diameters is analysed in detail. By using the bond valence sum approach we demonstrate that the types and geometry of most of hydrogen bonds created within the pores manifest a structural evolution from distorted hydrogen bonds inherent to small pores (∼7 Å) to typical hydrogen bonds observed by us in larger pores (∼15 Å) which are consistent with those observed in a wide database of inorganic crystals.

Nanostructured Silicon as Potential Anode Material for Li-Ion Batteries

Commercial micrometer silicon (Si) powder was investigated as a potential anode material for lithium ion (Li-ion) batteries. The characterization of this powder showed the mean particle size of approx.75.2 nm, BET surface area of 10.6 m^2/g and average pore size of 0.56 nm. Its band gap was estimated to 1.35 eV as determined using UV-Vis diffuse reflectance spectra. In order to increase the surface area and porosity which is important for Li-ion batteries, the starting Si powder was ball-milled and threatened by metal-assisted chemical etching. The mechanochemical treatment resulted in decrease of the particle size from 75 nm to 29 nm, an increase of the BET surface area and average pore size to 16.7 m²/g and 1.26 nm, respectively, and broadening of the X-ray powder diffraction (XRD) lines. The XRD patterns of silver metal-assisted chemical etching (MACE) sample showed strong and narrow diffraction lines typical for powder silicon and low-intensity diffraction lines typical for silver. The metal-assisted chemical etching of starting Si material resulted in a decrease of surface area to 7.3 m²/g and an increase of the average pore size to 3.44 nm. These three materials were used as the anode material in lithium-ion cells, and their electrochemical properties were investigated by cyclic voltammetry and galvanostatic charge-discharge cycles. The enhanced electrochemical performance of the sample prepared by MACE is attributed to increase in pore size, which are large enough for easy lithiation. These are the positive aspects of the application of MACE in the development of an anode material for Li-ion batteries.

Silicon Electrochemistry in Molten Salts

Silicon electrochemistry has the potential to advance sustainable energy solutions by offering environmentally friendly and secure technologies that can contribute to the low-carbon economy. Electrochemical methods use electrons directly as reducing agents, eliminating the need for harmful chemicals and offering simpler, one-step, process control. Silicon itself is the second most abundant element in the earth's crust, is nontoxic, and is a robust material offering high efficiencies in solar photovoltaics. As such, silicon currently dominates the solar energy market and could continue to do so for the next few decades. This review summarizes recent achievements in the molten salt electrochemistry of silicon, highlighting subjects of technological significance such as the production of silicon by silica electro-deoxidation, the formation of photoactive layers, silicon electrorefining, and the synthesis of semiconductors as well as nanostructures for energy storage applications. The review highlights future opportunities and challenges such as the production of highly pure silicon, the creation of carbon-free anodes for oxygen production, and silicon electrodeposition from gaseous precursors.

In-Fiber Production of Laser-Structured Stress-Mediated Semiconductor Particles

The ability to generate stressed semiconductor particles is of great importance in the development of tunable semiconductor and photonic devices. However, existing methods including both bottom-up synthesis and top-down fabrication for producing semiconductor particles are inherently free of stress effects. Here, we report a simple approach to generate controllable stress effects on both encapsulated and free-standing semiconductor particles using laser-structured in-fiber materials engineering. The physical mechanism of thermally induced in-fiber built-in stress is investigated, and the feasibility of precisely tuning the stress state during the particle formation is experimentally demonstrated by controlling the laser treatment. Gigapascal-level built-in stress, which is a sufficiently strong stimulus to enable inelastic deformations on the fabricated semiconductor particles, has been achieved via this approach. Both encapsulated and free-standing stressed semiconductor particles are generated for a wide range of in-fiber and out-fiber optoelectronic and biomedical applications.

Silicon-Nanographite Aerogel-Based Anodes for High Performance Lithium Ion Batteries

To increase the energy storage density of lithium-ion batteries, silicon anodes have been explored due to their high capacity. One of the main challenges for silicon anodes are large volume variations during the lithiation processes. Recently, several high-performance schemes have been demonstrated with increased life cycles utilizing nanomaterials such as nanoparticles, nanowires, and thin films. However, a method that allows the large-scale production of silicon anodes remains to be demonstrated. Herein, we address this question by suggesting new scalable nanomaterial-based anodes. Si nanoparticles were grown on nanographite flakes by aerogel fabrication route from Si powder and nanographite mixture using polyvinyl alcohol (PVA). This silicon-nanographite aerogel electrode has stable specific capacity even at high current rates and exhibit good cyclic stability. The specific capacity is 455 mAh g⁻¹ for 200^th cycles with a coulombic efficiency of 97% at a current density 100 mA g⁻¹.

In Situ TEM of Phosphorus-Dopant-Induced Nanopore Formation in Delithiated Silicon Nanowires

Through in situ transmission electron microscopy (TEM) observation, we report the behaviors of phosphorus (P)-doped silicon nanowires (SiNWs) during electrochemical lithiation/delithiation cycling. Upon lithiation, lithium (Li) insertion causes volume expansion and formation of the crystalline Li₁₅Si₄ phase in the P-doped SiNWs. During delithiation, vacancies induced by Li extraction aggregate gradually, leading to the generation of nanopores. The as-formed nanopores can get annihilated with Li reinsertion during the following electrochemical cycle. As demonstrated by our phase-field simulations, such first-time-observed reversible nanopore formation can be attributed to the promoted lithiation/delithiation rate by the P dopant in the SiNWs. Our phase-field simulations further reveal that the delithiation-induced nanoporous structures can be controlled by tuning the electrochemical reaction rate in the SiNWs. The findings of this study shed light on the rational design of high-power performance Si-based anodes.

A hierarchical porous architecture of silicon@TiO2@carbon composite novel anode materials for high performance Li-ion batteries

The excellent electrochemical properties are attributed to the synergistic action of hierarchical porous TiO₂ and carbon layers.

A Mini Review: Recent Advances in Surface Modification of Porous Silicon

Porous silicon has been utilized within a wide spectrum of industries, as well as being used in basic research for engineering and biomedical fields. Recently, surface modification methods have been constantly coming under the spotlight, mostly in regard to maximizing its purpose of use. Within this review, we will introduce porous silicon, the experimentation preparatory methods, the properties of the surface of porous silicon, and both more conventional as well as newly developed surface modification methods that have assisted in attempting to overcome the many drawbacks we see in the existing methods. The main aim of this review is to highlight and give useful insight into improving the properties of porous silicon, and create a focused description of the surface modification methods.

Systematic evaluation of inorganic salts as a heat sink for the magnesiothermic reduction of silica

In this study, the effectivity of a series of inorganic salts, sodium chloride, calcium chloride, magnesium chloride, potassium chloride, and sodium bromide as heat sinks during magnesiothermic reduction of silica to porous silicon was investigated. The salts were chosen based on cost, thermal stability, ability to remain chemically inert during the reduction process, and ease of removal after the reaction. The structural integrity of the spherical porous silicon nanoparticles was observed using scanning electron microscopy, the surface area was determined via nitrogen adsorption experiments, and the crystallite size was determined using powder X-ray diffraction analysis; together, these were used to determine the efficacy of each salt. The ability of a salt to act as an effective heat sink was found to be highly correlated and principally dependent on the heat capacity of the salt. Calcium chloride was found to be the most effective heat sink overall among the five heat sinks investigated here.

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.

Lithium effect on the electronic properties of porous silicon for energy storage applications: a DFT study

Theoretical studies on the effect of Li on the electronic properties of porous silicon are still scarce; these studies could help us in the development of Li-ion batteries of this material which overcomes some limitations that bulk silicon has. In this work, the effect of interstitial and surface Li on the electronic properties of porous Si is studied using the first-principles density functional theory approach and the generalised gradient approximation. The pores are modeled by removing columns of atoms of an otherwise perfect Si crystal, dangling bonds of all surfaces are passivated with H atoms, and then Li is inserted on interstitial positions on the pore wall and compared with the replacement of H atoms with Li. The results show that the interstitial Li creates effects similar to n-type doping where the Fermi level is shifted towards the conduction band with band crossings of the said level thus acquiring metallic characteristics. The surface Li introduces trap-like states in the electronic band structures which increase as the number of Li atom increases with a tendency to become metallic. These results could be important for the application of porous Si nanostructures in Li-ion batteries technology.