Minimized Volume Expansion in Hierarchical Porous Silicon upon Lithiation

Facile Biogenic synthesis of Europium doped lanthanum silicate nanoparticles as novel supercapacitor electrodes for efficient energy storage applications

In this work, we demonstrated for the first time, use of Europium doped lanthanum silicate nanoparticles (LS NPs) as electrodes for supercapacitor applications. Europium (Eu3+) doped (5 mol%) LS NPs were synthesized by green solution combustion method using Mexican mint leaf extracts. Various analytical techniques such as High-Resolution Transmission Electron Microscopy (HRTEM), Selected Area Diffraction (SAED), Powder X-ray Diffraction (PXRD), Fourier Transform Infra-Red Spectroscopy (FTIR) and Diffuse Reflectance Spectroscopy (DRS) techniques were used to confirm the morphological and structural characteristics of the synthesized nanoparticles. The HRTEM and SAED patterns confirms the formation of NPs having agglomerated structure with a particle size less than 50 nm. The PXRD patterns reveals crystalline cubic structure for the NPs. Further, the FT-IR spectra reveal the successful doping of Europium in Lanthanum Silicate NPs. The DRS (Diffuse Reflectance Spectroscopy) studies confirm the reduced band gap for Europium (Eu3+) doped (5 mol%) LS NPs. Cyclic voltametric and electrochemical impedance spectroscopy experiments were performed in an alkaline medium to compare the electrochemical activity of Eu3+ doped LS NPs with that of their undoped counterpart. The Eu3+ doped (5 %) LS NPs electrodes attained a specific capacitance of 373.3 Fg-1 at a current density of 0.5 Ag-1 in comparison to pure LS NPs which is about 267 Fg-1. The long-term stability of the Eu3+ doped (5 %) LS NPs electrodes show excellent stability up to 4000 cycles of operation in comparison pure LS NPs electrodes. Doping of Eu3+ had a favourable effect on the conductivity and electrochemical activity of LS NPs. Due to favourable green combustion synthesis, superior electrochemical performance, these Eu3+ doped LS NPs could be potential materials for new generation supercapacitors in energy storage applications.

Densification of Alloying Anodes for High Energy Lithium-Ion Batteries: Critical Perspective on Inter- Versus Intra-Particle Porosity

High Li-storage-capacity particles such as alloying-based anodes (Si, Sn, Ge, etc.) are core components for next-generation Li-ion batteries (LIBs) but are crippled by their intrinsic volume expansion issues. While pore pre-plantation represents a mainstream solution, seldom do this strategy fully satisfy the requirements in practical LIBs. One prominent issue is that porous particles reduce electrode density and negate volumetric performance (Wh L-1) despite aggressive electrode densification strategies. Moreover, the additional liquid electrolyte dosage resulting from porosity increase is rarely noticed, which has a significant negative impact on cell gravimetric energy density (Wh kg-1). Here, the concept of judicious porosity control is introduced to recalibrate existing particle design principles in order to concurrently boost gravimetric and volumetric performance, while also maintaining the battery's cycle life. The critical is emphasized but often neglected role that intraparticle pores play in dictating battery performance, and also highlight the superiority of closed pores over the open pores that are more commonly referred to in the literature. While the analysis and case studies focus on silicon-carbon composites, the overall conclusions apply to the broad class of alloying anode chemistries.

Reduced Volume Expansion of Micron-Sized SiOx via Closed-Nanopore Structure Constructed by Mg-Induced Elemental Segregation

The inherently huge volume expansion during Li uptake has hindered the use of Si-based anodes in high-energy lithium-ion batteries. While some pore-forming and nano-architecting strategies show promises to effectively buffer the volume change, other parameters essential for practical electrode fabrication, such as compaction density, are often compromised. Here we propose a new in situ Mg doping strategy to form closed-nanopore structure into a micron-sized SiOx particle at a high bulk density. The doped Mg atoms promote the segregation of O, so that high-density magnesium silicates form to generate closed nanopores. By altering the mass content of Mg dopant, the average radii (ranged from 5.4 to 9.7 nm) and porosities (ranged from 1.4 % to 15.9 %) of the closed pores are precisely adjustable, which accounts for volume expansion of SiOx from 77.8 % to 22.2 % at the minimum. Benefited from the small volume variation, the Mg-doped micron-SiOx anode demonstrates improved Li storage performance towards realization of a 700-(dis)charge-cycle, 11-Ah-pouch-type cell at a capacity retention of >80 %. This work offers insights into reasonable design of the internal structure of micron-sized SiOx and other materials that undergo conversion or alloying reactions with drastic volume change, to enable high-energy batteries with stable electrochemistry.

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.

Wafer-Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle-Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

Many biological materials exhibit a multiscale porosity with small, mostly nanoscale pores as well as large, macroscopic capillaries to simultaneously achieve optimized mass transport capabilities and lightweight structures with large inner surfaces. Realizing such a hierarchical porosity in artificial materials necessitates often sophisticated and expensive top-down processing that limits scalability. Here, an approach that combines self-organized porosity based on metal-assisted chemical etching (MACE) with photolithographically induced macroporosity for the synthesis of single-crystalline silicon with a bimodal pore-size distribution is presented, i.e., hexagonally arranged cylindrical macropores with 1 µm diameter separated by walls that are traversed by pores 60 nm across. The MACE process is mainly guided by a metal-catalyzed reduction-oxidation reaction, where silver nanoparticles (AgNPs) serve as the catalyst. In this process, the AgNPs act as self-propelled particles that are constantly removing silicon along their trajectories. High-resolution X-ray imaging and electron tomography reveal a resulting large open porosity and inner surface for potential applications in high-performance energy storage, harvesting and conversion or for on-chip sensorics and actuorics. Finally, the hierarchically porous silicon membranes can be transformed structure-conserving by thermal oxidation into hierarchically porous amorphous silica, a material that could be of particular interest for opto-fluidic and (bio-)photonic applications due to its multiscale artificial vascularization.

Constructing practical micron silicon anodes via a homogeneous and robust network binder induced by a strong-affinity inorganic oligomer

Designing robust binders has been demonstrated to be an effective and facile strategy to stabilize Si anodes. However, the binders that performed well for Si nanoparticles are not applicable for low-cost and accessible Si micron-powders. Hence, a novel binder design strategy is still greatly required for practical micron-Si anodes. Herein, a robust water-based network binder (named as c-PTP-Alg) has been designed via coupling potassium tripolyphosphate (PTP) inorganic oligomer with alginic acid (Alg) organic macromolecule. Owing to the unique structure of PTP, a network with high mechanical resistance can be constructed in c-PTP-Alg binder via strong ion-dipole interactions. Moreover, the highly soluble and dispersed PTP inorganic oligomer in water prevents the organic macromolecule from aggregation. This induces a homogeneous texture in the c-PTP-Alg binder, which enables the polar groups in the composite binder to anchor micron-Si particles efficiently. Therefore, by simply applying the c-PTP-Alg binder, a significantly improved electrochemical performance of micron-Si anode with a high reversible capacity of 1599.9 mAh g^-1 after 100 cycles at 3000 mA g^-1 has been obtained. More specially, the high-energy-density Si||S-PAN full cells have also been constructed, showing the practical application prospect of the c-PTP-Alg binder.

Ultrahigh Lithium Storage Capacity of Al2C Monolayer in a Restricted Multilayered Growth Mechanism

Designing anode materials with high lithium specific capacity is crucial to the development of high energy density lithium (ion) batteries. Herein, a distinctive lithium growth mechanism, namely, the restricted multilayered growth for lithium, and a strategy for lithium storage are proposed to achieve a balance between ultrahigh specific capacity and the need to avert uncontrolled dendritic growth of lithium. In particular, based on first-principles computation, we show that the Al₂C monolayer with a planar tetracoordinate carbon structure can be an ideal platform for realizing the restricted multilayered growth mechanism as a two-dimensional (2D) anode material. Furthermore, the Al₂C monolayer exhibits the ultrahigh specific capacity of lithium of 4059 mAh/g, yet with a low diffusion barrier of 0.039-0.17 eV and low open circuit voltage in the range of 0.002-0.34 V. These novel properties render the Al₂C monolayer a promising anode material for future lithium (ion) batteries. Our study also offers a design of promising 2D anode materials with a high specific capacity, fast lithium-ion diffusion, and safe lithium storage.

Characterization of Mechanochemical Modification of Porous Silicon with Arginine

Mechanochemistry initiated the reaction of hydrogen-terminated porous silicon (H/por-Si) powder with arginine. Samples were analyzed using Fourier-transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), zeta potential, scanning electron microscopy (SEM), and photoluminescence (PL) spectroscopy. Arginine, which was physisorbed onto the surface of por-Si, blue-shifted the peak PL intensity from ~630 nm for the H/por-Si to ~565 nm for arginine-coated por-Si. Grinding for 4 h reduced >80% of the initially 2–45 µm particles to <500 nm, but was observed to quench the PL. With appropriate rinsing and centrifugation, particles in the 100 nm range were isolated. Rinsing ground powder with water was required to remove the unreacted arginine. Without rinsing, excess arginine induced the aggregation of passivated particles. However, water reacted with the freshly ground por-Si powder producing H2. A zeta potential of +42 mV was measured for arginine-terminated por-Si particles dispersed in deionized water. This positive value was consistent with termination such that NH2 groups extended away from the surface. Furthermore, this result was confirmed by FTIR spectra, which suggested that arginine was bound to silicon through the formation of a covalent Si–O bond.

Stabilizing a Si Anode via an Inorganic Oligomer Binder Enabled by Robust Polar Interfacial Interactions

Exploiting macromolecule binders has been demonstrated as an effective approach to stabilize a Si anode with a huge volume change. The macromolecule polymer binders with vast intra/intermolecular interactions lead to an inferior dispersion of binders on a Si active material. Herein, a potassium triphosphate (PTP) inorganic oligomer was exploited as a robust binder to alleviate the problem of capacity fading in Si-based electrodes. PTP has abundant P-O^- bonds and P═O bonds, which can form strong ion-dipolar and dipolar-dipolar forces with a hydroxylated Si surface (Si-OH). Particularly, the PTP inorganic oligomer has a short-chain structure and high water solubility, resulting in a superior dispersion of the PTP binder on Si nanoparticles (nano-Si) to effectively enhance the mechanical stability of Si-based electrodes. Hence, the as-prepared Si-based anode exhibits obviously improved electrochemical performance, delivering a charge capacity of 1279.7 mAh g^-1 after 300 cycles at 800 mA g^-1 with a high capacity retention of 72.7%. Moreover, using the PTP binder, a dense Si anode can be achieved for high volumetric energy density. The success of this study shows that the PTP inorganic oligomer as a binder has great significance for future advanced binder research.

Metal-Assisted Catalytic Etching (MACE) for Nanofabrication of Semiconductor Powders

Electroless etching of semiconductors has been elevated to an advanced micromachining process by the addition of a structured metal catalyst. Patterning of the catalyst by lithographic techniques facilitated the patterning of crystalline and polycrystalline wafer substrates. Galvanic deposition of metals on semiconductors has a natural tendency to produce nanoparticles rather than flat uniform films. This characteristic makes possible the etching of wafers and particles with arbitrary shape and size. While it has been widely recognized that spontaneous deposition of metal nanoparticles can be used in connection with etching to porosify wafers, it is also possible to produced nanostructured powders. Metal-assisted catalytic etching (MACE) can be controlled to produce (1) etch track pores with shapes and sizes closely related to the shape and size of the metal nanoparticle, (2) hierarchically porosified substrates exhibiting combinations of large etch track pores and mesopores, and (3) nanowires with either solid or mesoporous cores. This review discussed the mechanisms of porosification, processing advances, and the properties of the etch product with special emphasis on the etching of silicon powders.

Architectural Engineering Achieves High-Performance Alloying Anodes for Lithium and Sodium Ion Batteries

Tremendous efforts have been dedicated to the development of high-performance electrochemical energy storage devices. The development of lithium- and sodium-ion batteries (LIBs and SIBs) with high energy densities is urgently needed to meet the growing demands for portable electronic devices, electric vehicles, and large-scale smart grids. Anode materials with high theoretical capacities that are based on alloying storage mechanisms are at the forefront of research geared towards high-energy-density LIBs or SIBs. However, they often suffer from severe pulverization and rapid capacity decay due to their huge volume change upon cycling. So far, a wide variety of advanced materials and electrode structures are developed to improve the long-term cyclability of alloying-type materials. This review provides fundamentals of anti-pulverization and cutting-edge concepts that aim to achieve high-performance alloying anodes for LIBs/SIBs from the viewpoint of architectural engineering. The recent progress on the effective strategies of nanostructuring, incorporation of carbon, intermetallics design, and binder engineering is systematically summarized. After that, the relationship between architectural design and electrochemical performance as well as the related charge-storage mechanisms is discussed. Finally, challenges and perspectives of alloying-type anode materials for further development in LIB/SIB applications are proposed.

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

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

Response of Photoluminescence of H-Terminated and Hydrosilylated Porous Si Powders to Rinsing and Temperature

The photoluminescence (PL) response of porous Si has potential applications in a number of sensor and bioimaging techniques. However, many questions still remain regarding how to stabilize and enhance the PL signal, as well as how PL responds to environmental factors. Regenerative electroless etching (ReEtching) was used to produce photoluminescent porous Si directly from Si powder. As etched, the material was H-terminated. The intensity and peak wavelength were greatly affected by the rinsing protocol employed. The highest intensity and bluest PL were obtained when dilute HCl(aq) rinsing was followed by pentane wetting and vacuum oven drying. Roughly half of the hydrogen coverage was replaced with –RCOOH groups by thermal hydrosilylation. Hydrosilylated porous Si exhibited greater stability in aqueous solutions than H-terminated porous Si. Pickling of hydrosilylated porous Si in phosphate buffer was used to increase the PL intensity without significantly shifting the PL wavelength. PL intensity, wavelength and peak shape responded linearly with temperature change in a manner that was specific to the surface termination, which could facilitate the use of these parameters in a differential sensor scheme that exploits the inherent inhomogeneities of porous Si PL response.

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.

Porosity controlled synthesis of nanoporous silicon by chemical dealloying as anode for high energy lithium-ion batteries

Silicon is regarded as the most promising electrode material to meet the high-capacity demand for lithium-ion batteries (LIBs). Nevertheless, the large volume expansion during charging/discharging process restricts its practical application. In this report, a facile chemical dealloying method is conducted to prepare porous silicon materials from Al-Si alloys with different proportions at ambient temperature. The porosity of anode materials could buffer the huge volume change of Si anode and enhance the ion transport. Finally, the optimized Si20 sample delivers a capacity of 1662 mAh g^-1 after 145 cycles at 500 mA g^-1 and a high rate capability up to 908 mAh g^-1 at 5000 mA g^-1.