Pristine Si Research Articles

Silicon Nanoparticles with a Polymer-Derived Carbon Shell for Improved Lithium-Ion Batteries: Investigation into Volume Expansion, Gas Evolution, and Particle Fracture

Silicon (Si) and composites thereof, preferably with carbon (C), show favorable lithium (Li) storage properties at low potential, and thus hold promise for application as anode active materials in the energy storage area. However, the high theoretical specific capacity of Si afforded by the alloying reaction with Li involves many challenges. In this article, we report the preparation of small-size Si particles with a turbostratic carbon shell from a polymer precoated powder material. Galvanostatic charge/discharge experiments conducted on electrodes with practical loadings resulted in much improved capacity retention and kinetics for the Si/C composite particles compared to physical mixtures of pristine Si particles and carbon black, emphasizing the positive effect that the core–shell-type morphology has on the cycling performance. Using in situ differential electrochemical mass spectrometry, pressure, and acoustic emission measurements, we gain insights into the gassing behavior, the bulk volume expansion, and the mechanical degradation of the Si/C composite-containing electrodes. Taken together, our research data demonstrate that some of the problems of high-content Si anodes can be mitigated by carbon coating. Nonetheless, continuous electrolyte decomposition, particle fracture, and electrode restructuring due to the large volume changes during battery operation (here, ∼170% in the voltage range of 600–30 mV vs Li+/Li) remain as serious hurdles toward practical implementation.

Biohybridization of Supported Gold Nanoassemblies on Silicon

Understanding the molecular interactions of bio-organic molecules with metal nanoassemblies on a semiconductor surface is important to developing potential applications involving hybrid bio-organic metal interfaces. Here, we provide the first study of room-temperature growth evolution of l-cysteine on three notable Au nanoassemblies supported on the Si(111)7×7 surface. Our results indicate unidentate and/or bidentate arrangement of adsorbed cysteine on the Si substrate through Si–N and/or Si–S linkages, while in coexistence with the supported Au monomers and dimers. Similar to thiol-containing molecules adsorbed on other noble metals, cysteine chemisorbs via the S atom in neutral form on the supported Au nanocrystallite film. On the supported gold honeycomb nanonetwork, cysteine undergoes unidentate chemisorption through the thiol group with Au atoms and through the amino group with Si adatoms, which enables the remaining free functional groups to selectively bond with different incoming molecules. Instead of the “universal” three-stage growth found for cysteine adsorption on a pristine Si(111)7×7 surface, we observe the two-stage growth of cysteine on the supported gold honeycomb nanonetwork (i.e., without a transitional layer), similar to that found on a gold single-crystal surface. The formation of the ultrathin gold-silicide layer (honeycomb) has effectively transformed the semiconductor surface to a metal-like surface.

Broadband antireflection property of conformally grown zinc tin oxide thin films on nanorippled- and nanofaceted-Si substrates

In this paper, we show the broadband antireflection property of radio frequency sputter-grown zinc tin oxide (ZTO) thin films grown on ion-beam fabricated self-organized nanoscale rippled- (R-Si) and faceted-Si (F-Si) substrates. For the sake of comparison, ZTO film has been deposited simultaneously on pristine-Si (P-Si) substrate as well. X-ray diffraction studies show the amorphous nature of the films. Atomic force microscopic studies reveal the conformal growth of ZTO thin films (granular in nature) on both R- and F-Si substrates (up to 60 nm). Surface reflectance measurements show a large reduction in the average broadband (300–800 nm) surface reflectance (at 550 nm) from 43.9% for P-Si substrate to 42% and 32% for R-Si and F-Si substrate, respectively. In addition, we show a further reduction in the surface reflectance following the growth of ZTO overlayers (in the range of 15–60 nm) on P-Si, R-Si, and F-Si substrates. As a matter of fact, there is a continuous reduction in surface reflectance up to a thickness of 60 nm, beyond which the reflectance minimum starts increasing. The large reduction in the surface reflectance of ZTO coated P-, R-, and F-Si substrates are attributed to the graded refractive index of the heterostructures driven by the surface morphology of the substrates and the different average surface heights of the films. This study will be very useful for efficient ZTO-based optoelectronic devices and heterojunction solar cell applications.

Fabrication of TiOx–Si photoanode and its energetic photoelectrochemical performance

Pristine Si is oxidized to insulative SiO2 when it comes in contact with air and water. Covering it with a protection layer inhibits passivation of Si and significantly improves its photoelectrochemical performance. In this study, TiOx with gradient change of oxygen stoichiometry ratio (TiOx) was designed as a protection layer and fabricated via a chemical vapour deposition process in Ar flow under 400 °C for 1 min. The anaerobic atmosphere and short heating duration synergistically produced the ratio of O and Ti lower than two in the prepared film. XPS analysis suggested the existance of TiO2 only at the surface of TiOx film and Ti3+ and Ti2+ appeared successively with the increase of distance to the surface. The first advantage of lower-valence-state Ti and oxygen deficiency was to inhibit the oxidation of Si and to reduce electric resistance of the interface and the protection layer. The second advantage was to create a defect energy level under the conduction band of TiO2 which provided the possibility for holes in the valence band of Si to be transferred to this defect level. This tunnel like transfer enhanced the photogenerated charge separation and redox ability of TiOx–Si which brought a 3.25 folds enhancements in photocurrent density compared to that of stoichiometric TiO2–Si at 0 V (SCE) under simulated sunlight. This study highly motivates further research on transparent and conductive protection layer of Si photoelectrode.

Reaction-Ball-Milling-Driven Surface Coating Strategy to Suppress Pulverization of Microparticle Si Anodes.

In this work, we report a novel reaction-ball-milling surface coating strategy to suppress the pulverization of microparticle Si anodes upon lithiation/delithiation. By energetically milling the partially prelithiated microparticle Si in a CO2 atmosphere, a multicomponent amorphous layer composed of SiO x, C, SiC, and Li2SiO3 is successfully coated on the surface of Si microparticles. The coating level strongly depends on the milling reaction duration, and the 12 h milled prelithiated Si microparticles (BM12h) under a pressure of 3 bar of CO2 exhibit a good conformal coating with 1.006 g cm-3 of tap density. The presence of SiC remarkably enhances the mechanical properties of the SiO x/C coating matrix with an approximately 4-fold increase in the elastic modulus and the hardness values, which effectively alleviates the global volume expansion of the Si microparticles upon lithiation. Simultaneously, the existence of Li2SiO3 insures the Li-ion conductivity of the coating layer. Moreover, the SEI film formed on the electrode surface maintains relatively stable upon cycling due to the remarkably suppressed crack and pulverization of particles. These processes work together to allow the BM12h sample to offer much better cycling stability, as its reversible capacity remains at 1439 mAh g-1 at 100 mA g-1 after 100 cycles, which is nearly 4 times that of the pristine Si microparticles (381 mAh g-1). This work opens up new opportunities for the practical applications of micrometer-scale Si anodes.

The RBS analysis for a thin amorphous Si layer formed on clean and H-terminated Si(0 0 1) surfaces followed by medium-energy IBIEC treatments

We have investigated the crystallization process of an amorphous Si layer grown on a clean and hydrogen-terminated Si(0 0 1) surfaces by ion beam induced epitaxial crystallization (IBIEC). It is widely known that the amorphous/crystalline (a/c) interface should be atomically controlled to achieve the high-quality Si epitaxy. In this study, we quantitatively clarify the influence of interfacial H amount to the IBIEC rate of the deposited amorphous Si layer. We initially prepared a typical H-free pristine Si(0 0 1)-(2 × 1) surface and two kinds of chemically H-terminated surfaces of as-treated and sequentially annealed at 350 °C in ultrahigh vacuum, corresponding to high (dihydride) and low (monohydride) hydrogen coverage, respectively. An amorphous Si layer was then in-situ deposited on these three types of surfaces by an electron bombardment evaporator at room temperature. The samples were irradiated by 180-keV Ar+ ions with an ion fluence of 5 × 1015 ions cm−2 at 300–500 °C. The Rutherford backscattering (RBS) analysis showed that the deposited Si on the dihydride surface is not crystallized at these temperatures; whereas, that on the monohydride surface is crystallized by Ar+ irradiation at 500 °C. The crystallization on the MH surface was found at significantly higher temperature than that on the H-free clean surface. The behavior of the interfacial H atoms during IBIEC process is discussed by means of RBS analysis.

UV–visible light detection with TiO2 thin film deposited on chemically textured p-Si substrate

This paper reports a study on the photodetection properties of TiO2 thin film deposited on chemically textured p-Si substrate and is compared to those deposited on pristine p-Si substrate. The structural properties of both the heterostructures were investigated using XRD analysis. FESEM images confirmed the deposition of TiO2 thin film on chemically pyramidal textured Si substrate. The average total reflectance of the textured Si substrate was reached to ~ 7.6% in the wavelength range of 300–900 nm. It was further decreased to ~ 6.5% after the deposition of 55 nm thick TiO2 on the top of the textured Si substrate. A systematic study was carried out to correlate the structural, optical and electrical properties of both the heterostructures. The electrical parameters of the heterojunction diodes were measured and compared under dark and illuminated conditions using UV and solar simulated light. UV as well as visible light detection property of the heterostructure of TiO2 thin film deposited on pyramidal textured Si substrate was improved compared to the one deposited on pristine Si by the factor 2.0 and 1.86, respectively, under the bias of − 2 V.

Loading Amorphous NiMoO4–xSx Nanosheet Cocatalyst to Improve Performance of p-Silicon Wafer Photocathode

Considering its small band gap and appropriate conduction band level, Si is a suitable semiconductor for photocathodes in a photoelectrochemical (PEC) cell. Unfortunately, its PEC activity is far from desirable because of inferior stability, high recombination of photogenerated electron–hole pairs, and sluggish hydrogen evolution kinetics at the Si/electrolyte interface. The introduction of low-cost and earth-abundant non-noble metal cocatalysts onto the Si photocathode is a promising route to improve its PEC performance. Herein, we fabricate efficient and stable photocathodes by integrating non-noble metal NiMoO4–xSx nanosheets with planar p-Si wafers, along with ultrathin TiO2 as a protective and passivating film at the intermediate and outermost layer. An onset potential of 0.3 V (vs RHE) and a photocurrent density of 5.1 mA cm–2 at −0.5 V (vs RHE) are obtained for the optimal photocathode, representing significant enhancement compared to the pristine Si wafers. Moreover, long-term measurement demonstr...

Synthesis of Carbon Nanotubes Networks Grown on Silicon Nanoparticles as Li-Ion Anodes

Using chemical vapor deposition, we grew carbon nanotubes (CNTs) on the surface of Si nanoparticles (NPs) that were coated with a thin iron shell. We studied the CNT growth mechanisms and analyzed the influence of (1) varying annealing times and (2) varying growth times. We show that an initial annealing is necessary to reduce the iron oxide shell and to start the formation of Fe NPs and their consequent coarsening. We characterize the evolution of the catalyst morphology and its influence of the morphology and structure of the CNTs grown. We studied this nanocomposite of Si NPs interconnected by CNTs grown on them as anode material for Li-ion batteries. Compared to the pristine Si NPs, the Si-CNT nanocomposite brings an increase of 40% in specific capacity after 100 cycles at 1800 mA/gSi with a high stability and a very low capacity loss per cycle of 0.06%. The electrochemical performance demonstrates how efficient the CNT shell on the Si NP is to mitigate the usual failure mechanism of Si NPs. Thus, the...

Enhanced photoelectrochemical properties of copper-assisted catalyzed etching black silicon by electrodepositing cobalt

Black silicon (Si) photoelectrodes are promising for improving the performance of photoelectrochemical (PEC) water splitting. Here, we report the fabrication of p-black Si and n+p-black Si photocathodes via a controllable copper-assisted catalyzed etching method. The etching process affects only the topmost less than 200 nm of Si and is independent of the surface doping. The synergistic effects of the excellent light harvesting of the black Si and the improved charge transfer properties of the p-n junction boost the production and utilization of photogenerated carriers. The mean reflectance of the pristine Si samples is about 10% from 400 to 950 nm, while that of the black Si samples is reduced as low as 5%. In addition, the PEC properties of the n+p-black Si photocathode can be further enhanced by depositing a cobalt (Co) layer. Compared with the p-Si sample, the onset potential of the Co/n+p-black Si photocathode is positively shifted by 560 mV to 0.33 V vs. reversible hydrogen electrode and the saturation photocurrent density is increased from 22.7 to 32.6 mA/cm2. The design of the Co/n+p-black Si photocathode offers an efficient strategy for preparing PEC solar energy conversion devices.

Silicon Nanoparticles: Stability in Aqueous Slurries and the Optimization of the Oxide Layer Thickness for Optimal Electrochemical Performance.

In this study, silicon nanoparticles are oxidized in a controlled manner to obtain different thicknesses of SiO2 layers. Their stability in aqueous slurries as well as the effect of oxide layer thickness on the electrochemical performance of the silicon anodes is evaluated. Our results show that slightly increasing the oxide layer of silicon nanoparticles significantly improves the stability of the nanoparticles in aqueous slurries and does not compromise the initial electrochemical performance of the electrodes. A careful comparison of the rate and cycle performance between 400 °C treated Si nanoparticles and pristine Si nanoparticles shows that by treating the silicon nanoparticles in air for slightly increasing the oxide layer, improvement in both rate and cycle performance can be achieved.

Fundamental Understanding of the Effect of Polymer Binder on the Interfacial Properties of Si-Electrode Model Systems

Research and development efforts have been focused on improving the performance of existing Li-ion battery systems and developing new systems to various applications. Si based electrodes have high theoretical capacities (nearly 4200 mAh/g) compared to the widely used carbon-based electrodes (372 mAh/g), which makes them a promising candidate to replace graphite electrodes [1]. However, Si experiences substantial volume changes (~320%) during the charging and discharging processes, which lead to the problems of particle cracking, particle isolation, and electrode delamination. This, in turn, affects not only the SEI stability but also results in the loss of electrode integrity. In order to overcome the loss of electronic connectivity and mechanical integrity of the bulk electrode during extended charge-discharge cycles, it is critical to identify an effective polymer binder. Consequently, it is essential to understand the effect of polymer binder on the electrochemical performance for Si/C anodes. In the current study, we examine the adhesion properties and chemical stabilities of the binder before and during cycling. Thin films of the binder polymers are spin coated onto the B-doped Si (100) wafers to achieve the thicknesses of the same order of magnitude as those calculated for composite electrodes (20 nm). For comparison, thin films of the binder polymers are also spin coated on Cu substrate and analyzed. For adhesion measurement, saturated solutions of polymer binders in NMP (poly(1-pyrenemethyl methacrylate (PPy) and polyvinylidene difluoride (PVDF)) or H2O (poly (acrylic acid lithium (PAALi)) are sandwiched between two 1 cm2 Si or Si/Cu wafers by drop casting. To break down the cell components to Si, binder and electrolyte for electrochemical testing, model electrodes are prepared by spin coating PPy binder onto Si wafers. Electrochemical cycling is performed for pristine, and PPy coated Si (Si/PPy) wafers using 1.2 M LiPF6 in ethylene carbonate (EC): diethyl carbonate (DEC) (3:7 wt.%) solvent systems. The PPy film surface exposed to the electrolyte as well as the substrate/binder interface are analyzed in terms of chemical and morphology changes using various spectroscopic and microscopic techniques. PPy showed superior adhesion toward Si and Cu. The strong adhesion of the PPy to the Si surface provide improved interparticle connection vs. other binders. Electrochemical testing results of the Si and Si/PPy electrodes are shown in Figure 1. The cathodic current between 2 V and 0.3 V shows the presence of approximately ten times more electrolyte reduction on the PPy coated Si on the first sweep compared to pristine Si electrode. The charge consumed in this region (0.028 C/cm2) is much higher than the charge needed for complete decomposition of PPy (0.012 C/cm2) [2]. Moreover, ex-situ analysis by XAS and XPS depth profiling indicates the electrochemical stability of PPy upon cycling. The induced electronic conductivity of the PPy may exacerbate the electrolyte reduction reactions relative to the uncoated surface. Below 0.05 V, the cathodic current becomes significantly higher on the pristine Si due to the onset of lithiation of the silicon as confirmed by the corresponding delithiation anodic peaks. No similar anodic behavior was observed for the PPy coated Si electrode on the first and following cycles. However, electrode polartization due to charge and/or mass transport limitation through the PPy film can be responsible for this effect. The ex-situ FTIR analysis indicates the major products of the reduction reactions on the first few cycles have alkyl silane (SiOCH2CH3 and SiOCH3) functionality. We propose that these products come from the selective reduction of DEC at the Si/PPy interface. These reduction products do not function as SEI since they appear to show inferior passivating behavior of Si/PPy as compared to Si wafer and block Li+ insertion into the Si lattice. We have learned that, to be effective in a composite electrode, PPy needs to act as a “glue” between particles (Si or graphite) and not a “coating” of the Si particles, and electrode preparation should be tailored this function. By understanding the surface change and their significance regarding cell life, effective new binder candidates can be developed, which can lead to more efficient LIB systems. Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, under the Applied Battery Research for Transportation (ABR) Program and Award Number DE-EE0006443.

Extended Gate Field Effect Transistor Using GaN/Si Hybrids Nanostructures for pH Sensor

The pH sensor of an extended gate field effect transistor (EGFET) with gallium nitride/silicon hybrid nanostructure is fabricated and analyzed. Si nanowires (NWs) are fabricated via the Ag-assisted electroless etching technique and are then covered by GaN NWs through plasma-enhanced chemical vapor deposition (PECVD). The GaN nanostructure is synthesized by introducing gallium oxide (Ga2O3) and nitrogen (N2) for the growth of NWs. The GaN nanowires supply a larger surface area than that of the pristine Si NWs, where there is a better sensitivity for pH sensor. The GaN/Silicon hybrid sensors exhibit a sensitivity higher (50.4[Formula: see text]mV/pH) than that of pristine Si NWs sensors (41.2[Formula: see text]mV/pH). This GaN/Si hybrid pH sensor prepared by simple and low-cost method may be potentially applied for cheap biosensor devices.

Silicon Nanoparticles/Carbon Composites as Anodes for High-performance Lithium-ion Batteries

Silicon has emerged as the most promising anode candidate for next-generation lithium-ion batteries (LIBs) owing to its high theoretical specific capacity of 3579 mAh/g combined with a relatively low working potential and its natural abundance. However, the practical application of pristine Si in anodes is seriously hindered by its low intrinsic electrical conductivity and the large volume variation during charging and discharging. The resulting mechanical stress causes rapid pulverization of the silicon, and isolation and disconnection of the active materials from the current collector. These failure events mostly cause a rapid degeneration of the Si electrodes. In this study, Si nanoparticles (Si-NPs) were synthesized in a hot-wall reactor using monosilane as precursor. This process enables producing high purity Si NPs with a large production rate of up to 1 kg/h. Particle size, morphology, and crystallinity of the Si-NPs can be tuned towards optimized properties for battery applications by adjusting the synthesis parameters. In order to overcome the above mentioned failure scenarios of Si-NP-based anodes, Si/carbon nanocomposites such as Si-CNT, Si/graphene, Si-CNT/porous carbon, and Si-CNT/graphene composites have been developed to improve mechanical as well as electrical properties. As a highly promising product, the Si-CNT/ graphene nano-heterostructure demonstrates a high reversible initial capacity of 1665 mAh/g and very stable cycling performance over 500 cycles with a capacity decay of only 0.02% per cycle. Besides, the composite exhibits also a high-rate capability of 755 mAh/g (45.3% retention) at 10 C. These superior results imply that Si/carbon nano-heterostructures can be used for the development of high-performance lithium-ion batteries for durable and high-rate uses.

Topological Insulator Bi2Se3/Si-Nanowire-Based p-n Junction Diode for High-Performance Near-Infrared Photodetector.

Chemically derived topological insulator Bi2Se3 nanoflake/Si nanowire (SiNWs) heterojunctions were fabricated employing all eco-friendly cost-effective chemical route for the first time. X-ray diffraction studies confirmed proper phase formation of Bi2Se3 nanoflakes. The morphological features of the individual components and time-evolved hybrid structures were studied using field emission scanning electron microscope. High resolution transmission electron microscopic studies were performed to investigate the actual nature of junction whereas elemental distributions at junction, along with overall stoichiometry of the samples were analyzed using energy dispersive X-ray studies. Temperature dependent current-voltage characteristics and variation of barrier height and ideality factor was studied between 50 and 300 K. An increase in barrier height and decrease in the ideality factor were observed with increasing temperature for the sample. The rectification ratio (I+/I-) for SiNWs substrate over pristine Si substrate under dark and near-infrared (NIR) irradiation of 890 nm was found to be 3.63 and 10.44, respectively. Furthermore, opto-electrical characterizations were performed for different light power intensities and highest photo responsivity and detectivity were determined to be 934.1 A/W and 2.30 × 1013 Jones, respectively. Those values are appreciably higher than previous reports for topological insulator based devices. Thus, this work establishes a hybrid system based on topological insulator Bi2Se3 nanoflake and Si nanowire as the newest efficient candidate for advanced optoelectronic materials.

Doping of nano-crystalline silicon powders by mechanical alloying: the process and characterization

Silicon is the most used semiconductor material in photovoltaic applications. Doping of silicon with III and V group elements to synthesize p and n-type Si requires high temperature (~1000 °C). In the present studies, doping was accomplished in nanocrystalline Si powder at room temperature using mechanical alloying (MA) technique. Aqueous solution of boric acid or phosphoric acid in requisite amount for 50 ppm doping was added to the pristine Si powder and dried. Mechanical alloying of the dried Si powder was performed in the attrition mill in argon atmosphere. MA Si powders were then degassed to remove entrapped argon and other volatiles in a vacuum of 10−2 torr at 200 °C. Scanning electron microscopy and energy dispersive X-ray analysis were performed to study the morphology and composition of ball milled powder. XRD analysis was done to ascertain phases present and crystallite size in the ball milled powders. Micro-Raman results showed an asymmetric peak in terms of Fano type interference and due to phonon scattering, confirming the doping. Degree of crystallinity was studied by deconvoluting Raman peaks. Synchrotron XRF of phosphorus doped Si powder confirms phosphorus doping, in coherence with the results of Raman spectra. XPS spectrum confirms the presence of bounded boron in doped Si powder with binding energy of 186 eV. Charge carrier concentrations and hall mobilities were determined at room temperature. The carrier concentration was found to be matched with the dopants concentration added initially.

Simulation Protocol for Prediction of a Solid-Electrolyte Interphase on the Silicon-based Anodes of a Lithium-Ion Battery: ReaxFF Reactive Force Field.

We propose the ReaxFF reactive force field as a simulation protocol for predicting the evolution of solid-electrolyte interphase (SEI) components such as gases (C2H4, CO, CO2, CH4, and C2H6), and inorganic (Li2CO3, Li2O, and LiF) and organic (ROLi and ROCO2Li: R = -CH3 or -C2H5) products that are generated by the chemical reactions between the anodes and liquid electrolytes. ReaxFF was developed from ab initio results, and a molecular dynamics simulation with ReaxFF realized the prediction of SEI formation under real experimental conditions and with a reasonable computational cost. We report the effects on SEI formation of different kinds of Si anodes (pristine Si and SiOx), of the different types and compositions of various carbonate electrolytes, and of the additives. From the results, we expect that ReaxFF will be very useful for the development of novel electrolytes or additives and for further advances in Li-ion battery technology.

All-Silicon Switchable Magnetoelectric Effect through Interlayer Exchange Coupling.

The magnetoelectric (ME) effect originating from the effective coupling between electric field and magnetism is an exciting frontier in nanoscale science such as magnetic tunneling junction (MTJ), ferroelectric/piezoelectric heterojunctions etc. The realization of switchable ME effect under external electric field in d0 semiconducting materials of single composition is needed especially for all-silicon spintronics applications because of its natural compatibility with current industry. We employ density functional theory (DFT) to reveal that the pristine Si(111)-3×3 R30° (Si3 hereafter) reconstructed surfaces of thin films with a thickness smaller than eleven bilayers support a sizeable linear ME effect with switchable direction of magnetic moment under external electric field. This is achieved through the interlayer exchange coupling effect in the antiferromagnetic regime, where the spin-up and spin-down magnetized density is located on opposite surfaces of Si3 thin films. The obtained coefficient for the linear ME effect can be four times larger than that of ferromagnetic Fe films, which fail to have the reversal switching capabilities. The larger ME effect originates from the spin-dependent screening of the spin-polarized Dirac fermion. The prediction will promote the realization of well-controlled and switchable data storage in all-silicon electronics.

Molecular dynamics simulations of the first charge of a Li-ion-Si-anode nanobattery.

Rechargeable lithium-ion batteries are the most popular devices for energy storage but still a lot of research needs to be done to improve their cycling and storage capacity. Silicon has been proposed as an anode material because of its large theoretical capacity of ∼3600 mAh/g. Therefore, focus is needed on the lithiation process of silicon anodes where it is known that the anode increases its volume more than 300%, producing cracking and other damages. We performed molecular dynamics atomistic simulations to study the swelling, alloying, and amorphization of a silicon nanocrystal anode in a full nanobattery model during the first charging cycle. A dissolved salt of lithium hexafluorophosphate in ethylene carbonate was chosen as the electrolyte solution and lithium cobalt oxide as cathode. External electric fields are applied to emulate the charging, causing the migration of the Li-ions from the cathode to the anode, by drifting through the electrolyte solution, thus converting pristine Si gradually into Li14Si5 when fully lithiated. When the electric field is applied to the nanobattery, the temperature never exceeds 360 K due to a temperature control imposed resembling a cooling mechanism. The volume of the anode increases with the amorphization of the silicon as the external field is applied by creating a layer of LiSi alloy between the electrolyte and the silicon nanocrystal and then, at the arrival of more Li-ions changing to an alloy, where the drift velocity of Li-ions is greater than the velocity in the initial nanocrystal structure. Charge neutrality is maintained by concerted complementary reduction-oxidation reactions at the anode and cathode, respectively. In addition, the nanobattery model developed here can be used to study charge mobility, current density, conductance and resistivity, among several other properties of several candidate materials for rechargeable batteries and constitutes the initial point for further studies on the formation of the solid electrolyte interphase in the anode. Graphical Abstract Nanobattery: LiCoO2 cathode, electrolyte solution of 1M Li+PF6- in ethylene carbonate, and Si crystal anode, which changes its volume due to lithiation during the first charge.

Novel Si-CNT/polyaniline nanocomposites as Lithium-ion battery anodes for improved cycling performance

A novel nanocomposite consisting of gas-phased produced Si nanoparticles, carbon nanotubes (CNTs), and polyaniline (PANi) is developed as an anode material (Si-CNT/PANi) for lithium-ion batteries. This nanocomposite integrates the merits from its three components, where Si nanoparticles provide high capacity, CNTs act as an electrically conductive and mechanically flexible network, and PANi coating further enhances the electrical conductivity and protects the silicon structure. An anode made of this nanocomposite shows a high reversible capacity of 2430 mAh/g with good capacity retention over 500 cycles compared to pristine Si. The Si-CNT/PANi nanocomposite also demonstrated a high Coulombic efficiency and improved rate-capabilities.