Macroporous Si Research Articles

Sea Sand-Derived Magnesium Silicide as a Reactive Precursor for Silicon-Based Composite Electrodes of Lithium-Ion Battery

Recently, it has been clearly elucidated that nanostructured Si-based composites hybridized with protective and conductive materials can present enhanced electrochemical performance as anodes for Li-ion batteries (LIBs). One of remaining issues is to develop a sustainable and economic method to synthesize these composites on a large scale for industrial applications. Herein, we introduce a modified magnesiothermic reaction route to prepare the aforementioned Si-based composite electrodes using sea-sand derived Mg2Si as a reactive precursor. Owing to its reducibility and lability, Mg2Si can readily reduce group IVA oxides, such as Na2CO3, SiO2, GeO2, and SnO2, resulting in macroporous Si surrounded by the reduced forms of the counter reactants (C, Si, Ge, and Sn, respectively), some of which can be electrochemically attractive. Notably, the porous Si-based composite can be synthesized by a simple solid state reaction, so simplicity and scalability can be obtained. Also, the sea sand precursor is naturally–abundant; hence this process can be cost-effective, scalable, and sustainable. Porous Si@C composite can be synthesized from the modified magnesiothermic reaction using a sea sand-derived Mg2Si precursor, showing a specific capacity of 1000 mAh/g at 200th cycle. Potentially this process can be used for practical synthesis of Si-based composites.

In English

We investigate the conditions for increase of the photoluminescence of CdS nanocrystals in polyethyleneimine and polyethyleneimine with carbon multiwall nanotubes on macroporous silicon substrates. Macroporous silicon structures are made using the photoelectrochemical etching of n‑Si wafers. Macropores with diameter D p = 2¸5 mm, depth h p = 40¸120 mm and concentration N p = (1¸6)×10 6 cm ‑2 were formed. CdS nanocrystals of 1.8–2 nm in size are synthesized in reaction between Cd 2+ and HS - in a colloidal solution of polyethyleneimine in water. C arbon multiwall high purity nanotubes with submicron length and 20 nm diameter were produced by catalytic pyrolysis of unsaturated hydrocarbons. “Polymer-nanoparticles” coatings are deposited from a colloidal solution in water on single crystalline silicon, macroporous silicon and oxidized macroporous silicon. The maximal photoluminescence intensity of CdS nanocrystals is measured f or the oxidized macroporous silicon substrates with maximal electric field strength at the Si- SiO 2 interface . The photoluminescence quantum yield of CdS nanocrystals on the surface of oxidized macroporous silicon increases with time and reaches 28 %. Photoluminescence of polyethyleneimine with carbon multiwall nanotubes on macroporous silicon with microporous layer is 3–6 times more intense as compared with substrates c-Si, macroporous Si and oxidized macroporous Si due to a non-radiative proton recombination decrease at the silicon matrix boundary with microporous layer and nanocoating.

Enhancing light trapping of macroporous silicon by alkaline etching: application for the fabrication of black Si nanospike arrays

Extended alkaline etching of macroporous Si, produced by photoelectrochemical etching, influences the surface structures and their optical properties. We use this treatment for fabricating highly-absorbing arrays of sharp and crystalline nanospikes.

Fabrication of macroporous Si alloy anodes using polystyrene beads for lithium ion batteries

In this work, macroporous Si alloy anodes were manufactured using polystyrene (PS) spheres as a pore-forming agent. The PS spheres were easily eliminated by heat treatment, and the porosity and pore size of the electrodes were controllable by adjusting the content and size of the PS aggregates. Following heat treatment, the electrode packing density of the Si alloy anodes with 10–20 % PS decreased by 9–26 % due to removal of the PS, whereas the control electrode without PS displayed only a 1.3 % decrease. The porous electrode prepared using 10 or 15 % PS showed improved cyclic performances with capacities of 1110–1210 mAh g−1 after 100 cycles at 0.5 C. Although the macroporous Si alloy anode fabrication process using the PS beads reduced the electrode adhesion strength, the electrode performance improvements were attributed to the presence of macropores in the Si alloy electrode, which significantly reduced the changes in electrode thickness during lithiation/delithiation. The manufacture of macroporous electrodes using an appropriate PS content offers an effective strategy for applying Si-based anodes in advanced lithium ion batteries.

Light Trapping and Surface Passivation of Micron-Scaled Macroporous Blind Holes

We fabricate a blind hole surface texture by anodic etching of macroporous Si. The blind holes, i.e., pores that do not penetrate the wafer completely, have an average diameter of 2.7 μm, a distance of 4 μm, and a depth of 9 μm. This texture is capable of reducing the AM1.5G photon flux-weighted front reflectance to 1.5% without depositing an antireflection coating. The μm-feature size makes it a less fragile alternative to common nm-sized black silicon structures. We passivate the blind holes by atomic layer deposited ${\rm AlO}_{x}$ . The blind hole texture allows for a carrier lifetime of (2.2 ± 0.25) ms corresponding to an effective surface recombination velocity of (8 ± 1.5) cm/s with respect to the macroscopic front surface. A direct comparison of the optical performance and the surface passivation quality with a standard ${\rm SiN}_{x}$ -coated random pyramid surface shows that blind holes allow for a relative efficiency gain of (3 ± 0.2)% when applied, e.g., in an otherwise perfect back-contacted solar cell.

Anodes Based on Porous Silicon Films Using Polymer Electrolyte for Lithium-Ion Microbatteries

Today, the development of efficient storage systems is an important topic for electronic manufacturers. Indeed, with the arrival of new portable autonomous systems such as RFID Taggs, SmartCards, autonomous sensors, it’s important to develop a new energy storage compatible with these new technologies. Current Li-ion technologies are not compatible with these new technologies. This is due to the oversize in comparison with electronic device. Moreover, classical Li-ion batteries use liquid electrolyte. Liquid electrolyte is an important problem for safety (flammable, leakage). In this context, the development of low flammability and no volatile electrolytes to replace classical liquid electrolytes is then highly popular. One alternative electrolyte was proposed: polymer based electrolytes. Polymer electrolyte composed with PVdF/HFP and Pyrrolidinium bis(fluorosulfonyl)imide (P13FSI) was chosen for their good electrochemical, mechanical and transport properties. Concerning microbatteries, due to its high theoretical gravimetric capacity of 3579 mAh.g-1 at room temperature, silicon (Si) is a candidate to use in microbatteries. Bulk single crystalline silicon wafers are not suitable for a reliable negative electrode material because of strong volume changes that Si undergo during lithiation and delithiation reactions [1]. Nanostructured Si materials would be stable towards volumetric expansion and would limit crackings and pulverization during cycling, and hence improve the capacity retention [2]. We present an inexpensive and industrially scalable procedure to prepare porous silicon (pSi) based anodes. Meso or macroporous Si is prepared by electrochemical etching of a Si wafer to form a pSi layer, which is then simultaneous peeled-off and transferred onto a metallic current collector. Free standing and flexible films of porous silicon are thus achieved. Transferring a macroporous silicon layer onto a copper current collector follows these steps : 1) Electrochemical etching of Si wafer to achieve pores (Fig.1a) 2) Copper nanoparticles seeding by electroless deposition (Fig.1b) 3) Electroplating of a copper film 4) Mechanical peeling-off of the pSi/Cu film. SEM observation after peeling shows that the obtained pSi/Cu film surface is homogeneous (Fig.1c) and at higher magnification it is possible to descry the Cu seeding film at the bottom of the pores (inset of Fig.1c). The photograph of the final device (Fig.1d) shows, at a macroscopic scale, the homogeneity of the integrated pSi on the Cu current collector and the flexibility of the pSi/Cu film. Microbatteries were assembled by hot pressed polymer electrolyte on the pSi/Cu film. After that, lithium foil was put on it and sealed in button cell. In this communication, our results on the characterization and the use of polymer electrolyte with anodes based on porous silicon will be exposed. Thermal properties, ionic conductivity, diffusion coefficient of the electrolyte and electrochemical studies (galvanostatic cycling) will be exposed. A comparative study between polymer electrolyte and the corresponding ionic liquid (P13FSI) as ionic liquid electrolyte and their impact on the silicon morphology will be presented. 1. Y. He, X. Yu, G. Li, R. Wang, H. Li, Y. Wang, H. Gao, X. Huang, Journal of Power Sources, 216 131 (2012). 2. M. Ge, J. Rong, X. Fang and C. Zhou, Nanoletters, 12, 2318 (2012). Figure 1

Revisit of metallothermic reduction for macroporous Si: compromise between capacity and volume expansion for practical Li-ion battery

We revisit the metallothermic reduction process to synthesize shape-preserving macro-/nanoporous Si particles via aluminothermic and subsequent magnesiotheric reaction of porous silica particles. This process enables us to control the specific capacity and volume expansion of shape-preserving porous Si-based anodes. Two step metallothermic reactions have several advantages including a successful synthesis of shape-preserving Si particles, tunable specific capacity of as-synthesized Si anode, accommodation of a large volume change of Si by porous nature and alumina layers, and a scalable synthesis (hundreds of gram per batch). An optimized macroporous Si/Al2O3 composite anode exhibits a reversible capacity of ~1500mAhg−1 after 100 cycles at C/5 and a volume expansion of ~34% even after 100 cycles.

Control of interfacial layers for high-performance porous Si lithium-ion battery anode.

We demonstrate a facile synthesis of micrometer-sized porous Si particles via copper-assisted chemical etching process. Subsequently, metal and/or metal silicide layers are introduced on the surface of porous Si particles using a simple chemical reduction process. Macroporous Si and metal/metal silicide-coated Si electrodes exhibit a high initial Coulombic efficiency of ∼90%. Reversible capacity of carbon-coated porous Si gradually decays after 80 cycles, while metal/metal silicide-coated porous Si electrodes show significantly improved cycling performance even after 100 cycles with a reversible capacity of >1500 mAh g(-1). We confirm that a stable solid-electrolyte interface layer is formed on metal/metal silicide-coated porous Si electrodes during cycling, leading to a highly stable cycling performance.

Multi-functionalities of natural polysaccharide for enhancing electrochemical performance of macroporous Si anodes

Highly porous electrodes composed of natural polysaccharide act as a microporous binder and a carbon-coating source of micron-sized macroporous Si active materials was achieved. This highly porous Si-based anodes exhibit significantly improved electrochemical properties which show a high specific capacity (2010 mA h g−1) after 80 cycles.

Tunable charge transport through n-ZnO nanorods on Au coated macroporous p-Si

Diode-like and Ohmic charge transport through aligned ZnO nanorods on Au coated macroporous Si.

The nanostructure of the Si–Al eutectic and its use in lithium batteries

Abstract

First Biosourced Monolithic Macroporous SiC/C Composite Foams (Bio‐SiC/C(HIPE)) Bearing Unprecedented Heat Transport Properties

For the first time SiC/C composite foams were prepared from concentrated emulsions by carbothermal reduction of biosourced precursors combining sodium silicate by lignin at 1400 °C. The composition of the materials was determined by XRD, FTIR and Raman analyses. Their porous structure was characterized by SEM, mercury intrusion porosimetry, and nitrogen sorption, while their thermal properties were measured by TGA and dynamic DSC. Concerning their heat transport properties, we found out that when the starting lignin content was increased, the final C/Si ratio, the specific surface area and the heat diffusivity increased as well. Its unprecedented high values were attributed to a cooperative effect between radiative heat transfer and the presence of partially graphitized carbon.

Thin and flexible silicon anode based on integrated macroporous silicon film onto electrodeposited copper current collector

The integration of a macroporous Si (pSi) layer on an electrodeposited Cu current collector is proposed as a facile, low-cost and industrial-scalable procedure to elaborate efficient new anode material for lithium-ion batteries. The preparation process consists of i) formation of macropores on a Si wafer by electrochemical etching, ii) chemical deposition of Cu NPs on microstructured Si, iii) electroplating of a thick continuous Cu film, and iv) peeling of pSi–Cu film. The anode device is flexible and presents a total thickness, comprising the current collector, of 25 μm. SEM characterization shows the partial integration of the copper film inside the Si porous structure. Electrochemical impedance spectroscopy measurements showed this architecture enhances the electrical contact between the Cu current collector and the pSi due to the optimized interface of the two materials. A specific capacity of 1360 mAh g−1 in EC/DMC LiPF6 1 M is achieved at low galvanostatic discharge current (0.2 A g−1). Even during cycling at a high current density of 1.8 A g−1, the macroporous silicon anode was stable, demonstrating a specific capacity of 750 mAh g−1 twice as large as graphite based anodes.

(Invited) Applications of Porous Silicon to Multicrystalline Silicon Solar Cells: State of the Art

Porous Si is a potential candidate to be used as an antireflective coating or to texturize the front surface of multicrystalline silicon (mc-Si) wafers. Mesoporous Si was used as an antireflective coating and macroporous Si exhibiting light trapping properties replaced the alkaline or acidic texturization used in the photovoltaic industry. Mesoporous Si was either formed electrochemically or by stain etching. Macroporous Si was formed electrochemically in HF/organic solution or by metal-assisted etching. The properties of mesoporous and macroporous Si were optimized for mc-Si wafers and evaluated in large size solar cells.

Photoluminescence From Oxidized Macroporous Silicon: Nanoripples and Strained Silicon Nanostructures

We report synthesis of luminescent porous silicon (Si) having uniform macromorphology by sequential oxidation and etching of macroporous Si formed in a hydrofluoric acid and dimethylformamide solution. The macroporous template exhibits room-temperature photoluminescence (PL) with peaks in the blue/green region. Structural investigations reveal the formation of nanoripples and other strained nanostructured Si inside the pores of the oxidized samples. The room-temperature PL disappears, and the structure of the macropores changes on treating the oxidized templates with an alkaline solution. Based on these findings, we explicitly show that the origin of luminescence is due to the presence of nanoripples and strained Si nanostructures. Luminescent macroporous Si therefore reveals interesting physics but are untenable for application of light sources in optoelectronic devices.

Silicon–boron–carbon–nitrogen monoliths with high, interconnected and hierarchical porosity

Silicon–boron–carbon–nitrogen (Si–B–C–N) monoliths with high, interconnected and hierarchical porosity have been prepared by spark plasma sintering (SPS) of ordered mesoporous powders with a P6mm hexagonal symmetry at low temperature without any sintering additives. The ordered mesoporous Si–B–C–N powders derived from boron-modified polycarbosilazanes displayed a mesopore population centred on 4.6 nm, a total pore volume of 0.78 cm3 g−1 and a specific surface area of 544 m2 g−1. They have been partially sintered in the temperature range 800–1000 °C under nitrogen to lead to robust meso-/macroporous Si–B–C–N monoliths with surface areas of 123–171 m2 g−1, mesopore diameters centred on 6.2–6.5 nm and total pore volumes varying from 0.25 to 0.35 cm3 g−1 measured by nitrogen adsorption experiments. As-obtained crack-free Si–B–C–N monoliths displayed porosities from 59 to 69% and a relatively large pore size distribution as determined by helium pycnometry and confirmed by mercury porosimetry. TEM observations and low angle X-ray diffraction (LA-XRD) confirmed the formation of monoliths that maintained a mesoporosity organization in comparison to starting powders while SEM experiments showed a microstructure in which necks are formed around the area of contact between particles. With a thermal stability extending up to 1400 °C in flowing nitrogen and a heat conductivity of 0.647 W m−1 K−1 for the most porous component, these new materials display the necessary intrinsic properties required as porous supports working in a harsh environment.

Multilayer Etching for Kerf-free Solar Cells from Macroporous Silicon

Kerf-free techniques for subdividing a single thick crystalline Si wafer into a multitude of thin Si layers have a large potential for cost reductions. In this paper, we explore pore formation in Si for separating many 18μm-thick surface- textured layers from a thick wafer with a single etching process. We demonstrate the fabrication and separation of four macroporous Si layers in a single etching step. Generating many instead of single macroporous layers per etching step improves the economics of the macroporous Si process. We present our etching process that maintains the pore pattern defined by photolithography even after etching many absorber and separation layers.

Thin Crystalline Macroporous Silicon Solar Cells with Ion Implanted Emitter

We separate a (34 ± 2) μm-thick macroporous Si layer from an n-type Si wafer by means of electrochemical etching. The porosity is p = (26.2 ± 2.4) %. We use ion implantation to selectively dope the outer surfaces of the macroporous Si layer. No masking of the surface is required. The pores are open during the implantation process. We fabricate a macroporous Si solar cell with an implanted boron emitter at the front side and an implanted phosphorus region at the rear side. The short-circuit current density is 34.8mA cm–2 and the open-circuit voltage is 562mV. With a fill factor of 69.1% the cell achieves an energy-conversion efficiency of 13.5%.

Two-dimensional SixGe1−x films with variable composition made via multilayer colloidal template-guided ionic liquid electrodeposition

The binary alloy system Si(x)Ge(1-x) provides a continuous series of materials with gradually varying properties. In this paper, we report on a fundamental basis a method to make large-area macroporous Si(x)Ge(1-x) films with variable Ge content by electrodeposition in an ionic liquid, with SiCl(4) and GeCl(4) as precursors. The chemical composition of the products can be modified by changing the molar ratio of the precursors. Periodical macroporous Si(x)Ge(1-x) was made by a multilayer polystyrene (PS) template assembled as face-centered cubic lattice. Two-dimensional (2-D) Si(x)Ge(1-x) bowl-like and fishing-net structures can be obtained by applying different deposition temperatures. The results highlight the potential applications, including photonic bandgap and battery materials, as well as ultra-thin gratings, due to the effect of modification of light and improved tunability of composition, although Si(x)Ge(1-x) made by our method is sensitive to oxidation by air.

Optical characterization of ZnO nanopillars on Si and macroporous periodic Si structure

ZnO nanopillars were successfully grown using both the vapor-liquid-solid and the aqueous chemical growth methods on different substrates, such as quartz, n-, and p-type non-porous Si wafer (flat) and microporous periodic Si structure (MPSiS). Scanning electron microscopy was employed to compare sample morphologies. The absorption was calculated employing the GW0 method, based on the local density approximation, and with the projector augmented wave approach. Experiment and theory show a reasonable agreement when the shape of the optical absorption is considered. The measured absorption of ZnO nanopillars, on different substrates, is lower than that observed for ZnO films on quartz substrate, in the energy gap spectral range. A strong effect of MPSiS substrates on ZnO nanopillar properties is observed. The photoluminescence technique was also employed as an optical characterization.