Silicon Electrodes Research Articles

SECM as a Direct Method to Track Changes in Silicon Passivation

Silicon is a promising next-generation anode to increase energy density over current state of the art graphite anodes. Despite much progress in improving the cycling of silicon electrodes, the calendar life is still problematic. In this work, scanning electrochemical microscopy (SECM) was used to track the reactivity of the surface of a silicon thin film over time to better understand the role of the solid electrolyte interphase (SEI) during calendar aging. The passivation of silicon was found to decrease with increasing time and potential relative to Li/Li+. Along with the decrease in passivation with time, the homogeneity of passivation decreases. Despite some local “hot spots” of reactivity, within resolution limits (tip radius of 12.5 μm), the changes in passivation seemed to be global SEI failure rather than local (for example, cracking). In all cases, the delithiated silicon was less passivated than the lithiated silicon. When delithiated up to 1.5 V versus Li/Li+ the surface was found to be more reactive than the pristine silicon surface. This could potentially be the result of destabilization of the surface by SEI oxidation. When the cell was only delithiated up to 0.75 V versus Li/Li+, the surface was still passivating, but still less so than the lithiated surface. This indicates that the potential of the anode should be kept at or below ∼ 0.75 V Li/Li+ for half cell and full cell cycling to prevent decreasing SEI passivation.This research was supported by the U.S. Department of Energy’s Vehicle Technologies Office under the Silicon Consortium Project, directed by Brian Cunningham, and managed by Anthony Burrell. This work was conducted in part by the Alliance for Sustainable Energy, LLC, the manager and operator of the National Renewable Energy Laboratory for the U.S. Department of Energy (DOE) under Contract No. DE-AC36-08GO28308. Sandia National Laboratories is a multimission Laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

A novel multi-axis pendulum sandwich structure accelerometer

This paper proposes a new scheme of pendulum accelerometer with sandwich structure. In this scheme, the electrical signal on the mass is connected to the wafer surface through glass isolated through-silicon-via (TSV), so that the basic characteristics of the accelerometer can be obtained by wafer-level testing. Compared with the current commercial device process scheme, the packaging and testing cost of the device can be greatly reduced. The glass-in-silicon (GIS) encapsulation caps on both sides of the device are prepared by glass isolated TSV and GIS reflow process. By designing the shape and size of the silicon electrode area and glass area of the GIS caps, the parasitic capacitance between the pendulum structure and the fixed electrode is reduced. Another advantage of using TSV to extract the electrical signal of the mass is that the Z-axis inertial force can be detected when the pendulum structure is placed perpendicular to the direction of gravity acceleration. In addition, electrodes are grown on the side of the split device, and when the pendulum structure is placed parallel to the direction of gravity acceleration, the inertial force of the XY-axis can be detected. The test shows that when the range of the pendulum accelerometer based on this scheme is ±2 g, the noise density of the accelerometer is 42 μg √Hz−1 (X-axis), 40 μg √Hz−1 (Y-axis) and 27 μg √Hz−1 (Z-axis), and the bias instability is 6.6 μg (X-axis), 7.1 μg (Y-axis) and 6.8 μg (Z-axis), reaching the level of commercial devices.

Synthesis and characterization of metabolite-nanoparticle conjugates for use in metabolite biosensor systems

Detecting specific metabolites and measuring their changing levels in various biofluids has played a key role in understanding health and diagnosing disease. While most metabolite measurements are performed via spectroscopic (colorimetric, UV, mass spectrometry-MS, nuclear magnetic resonance) methods, it is also possible to perform metabolite detection and quantification via metabolite-antibody interactions, such as those used in competitive enzyme-linked immunosorbent assays (ELISAs), and/or impedance measurements. While metabolite-specific antibodies are available, conjugated-metabolites needed for metabolite-antibody detection are rare. Here we describe a general method that allows for the efficient conjugation of different classes of metabolites (acids, amines, and aromatic compounds) to gold nanoparticles and liposomes. We also describe a method for the efficient insertion of metabolite-lipid complexes into liposomes. We extensively characterized these conjugates, confirming their identity and composition, using a wide variety of analytical (thermal gravimetric analysis), spectroscopic (dynamic light scattering, MS) and microscopic (transmission electron microscopy-TEM, cryo-EM, high resolution EM) techniques. We also demonstrated that these metabolite conjugates can successfully bind to their metabolite-specific antibodies. We used scanning electron microscopy, atomic force microscopy, and ELISA to confirm the successful binding of these NPs-conjugates (N1-acetylspermine (AcSpm), hippuric acid and creatinine) to metabolite-specific antibodies attached to silicon wafers and electrodes. We believe the methods developed here are quite general and could be used in the development of a number of different types of metabolite biosensors and portable metabolite assays.

A coupled mechanical-electrochemical phase-field formulation for understanding the evolution of lithiated-silicon sponge

The unexpected formation of lithiated-silicon sponge during cyclic charge of lithium-ion batteries with silicon anodes has been observed in recent reports. The severe hollowing of silicon electrodes leads to an irreversible deformation and the generation of new surfaces, which greatly affects the structural integrity and cycling stability of the cells. However, the understanding of lithiated-silicon sponge still remains as a great challenge. In this work, a coupled mechanical-electrochemical model based on the phase-field method is proposed, which can describe the void evolution and the surface instability in Si electrodes. The model combines a description of the lithium diffusion, vacancy nucleation and annihilation, and an elastic-plastic constitutive model with concentration-dependent material properties. The simulation results reveal that the void within/on the silicon electrode tends to grow toward/along the electrode surface and toward other voids, leading to the electrode surface disruption and void merging, as well as the intra-electrode tunnel formation and electrolyte intrusion. Importantly, it is found that mechanical pressure can not only suppress the growth of a single void but also prevent the linkage and merging between multiple voids. The proposed theoretical framework and the findings of this work provide an in-depth understanding of the complicated mechanical-electrochemical coupling behavior of Si electrodes.

Surface Chemistry-Controlled SEI Layer on Silicon Electrodes by Regulating Electrolyte Decomposition.

Unstable solid electrolyte interface (SEI) layers induced by significant volume changes and subsequent side reactions at the interface have prevented Si anodes from practical application in lithium-ion batteries. The interface stability plays an important role in the electrochemical performance of Si electrodes. Here, we modify the interface of a Si electrode with ion-conductive poly(ethylene glycol) diglycidyl ether (PEGDE), which controls the electrolyte decomposition route and stabilizes the SEI layer. It enables the Si electrode to achieve a capacity of more than 1800 mAh g-1 at a current density of 2 A g-1, with a capacity retention of 77.25% after 300 cycles. The PEGDE-decorated Si electrode also shows greatly improved rate capability, with specific capacity up to 777 mAh g-1 even at 20 A g-1. We demonstrate that PEGDE decoration greatly increases the Li2CO3 ratio in the SEI layer, which improves the interface stability and Li+ conductivity and hence suppresses continuous electrolyte decomposition. As a result, the structural integrity of the Si particles is maintained and capacity fading is retarded. This work reveals that surface design can effectively regulate the SEI layer composition and improve interface stability, which is a promising strategy for Si-electrode manufacture.

The high performance lithium metal composite anode by excessive alloying process

With the rapid development of technology and the severe environmental crisis, improving the utilization and conversion rate of sustainable energy has become an important way to alleviate ecological and sustainable development. Therefore, the development of a new generation of anode materials with high specific capacity, long cycle life has become the primary task. Elements that can react with lithium and form alloy can be used as electrode materials, but these materials have limited capacity and poor electrochemical property. Lithium metal, as the ideal anode in lithium batteries, are restrained because of the dendrites formation. Herein, we introduced lithium metal composite anode by excessive alloying process, and to some extent, lithium dendrite formation and electrochemical issues are solved. The results show that lithium metal can be deposited evenly on the silver and silicon electrode surface, and both silver and silicon can enhance the coulombic efficiency and improve the electrochemical stability. In particular, silver-lithium composite electrodes can maintain 70 cycles with coulombic efficiency over 60 % which is twice as much as bare Cu electrodes. By this way, we make a new lithium metal composite anode structure and provide excessive alloying process to extend the application of commercial anode.

The Role of Silicate Enrichment on the Discharge Duration of Silicon-Air Batteries.

Silicon-air batteries are candidates for next generation batteries from non-critical raw materials. However, current silicon-air batteries with alkaline electrolytes suffer from premature termination of the discharge process. To understand this process, we investigated the correlation of dissolved silicon in the electrolyte and the discharge duration until passivation. The air- and Si-electrode could be excluded as the source of the voltage drop, while the concentration of silicates in the electrolyte was identified as the decisive factor. A low silicate concentration in the electrolyte was found to be crucial for a sustained discharge of silicon-air batteries with alkaline electrolytes and full consumption of the silicon electrode used in these conditions was shown.

Complementary Doped Source-Based Reconfigurable Schottky Diode as an Equivalence Logic Gate.

A complementary doped source-based reconfigurable Schottky diode (CDS-RSD) is proposed for the first time. Unlike other types of reconfigurable devices that have source and drain (S/D) regions with the same material, this has a complementary doped source region as well as a metal silicide drain region. Compared to three-terminal reconfigurable transistors, which have both the program gate and control gate, the proposed CDS-RSD does not have a control gate but only a program gate for reconfiguration operation. The drain electrode of the CDS-RSD is not only the output terminal of the current signal but also the input terminal of the voltage signal. Therefore, it is a reconfigurable diode based on high Schottky barriers for both the conduction band and valence band of silicon, which formed on the interface between the silicon and drain electrode. Therefore, the CDS-RSD can be regarded as the simplification of the reconfigurable field effect transistor structure on the premise of retaining the reconfigurable function. The simplified CDS-RSD is more suitable for the improvement of logic gate circuit integration. A brief manufacture process is also proposed. The device performance has been verified through device simulation. The performance of the CDS-RSD as a single-device two-input equivalence logic gate has also been investigated.

Interfacial Chemistry Effects in the Electrochemical Performance of Silicon Electrodes under Lithium-Ion Battery Conditions.

Understanding the solid electrolyte interphase (SEI) formation and (de)lithiation phenomena at silicon (Si) electrodes is key to improving the performance and lifetime of Si-based lithium-ion batteries. However, these processes remain somewhat elusive, and, in particular, the role of Si surface termination merits further consideration. Here, scanning electrochemical cell microscopy (SECCM) is used in a glovebox, followed by secondary ion mass spectrometry (SIMS) at identical locations to study the local electrochemical behavior and associated SEI formation, comparing Si (100) with a native oxide layer (SiOx /Si) and etched with hydrofluoric acid (HF-Si). HF-Si shows greater spatial electrochemical heterogeneity and inferior lithiation reversibility than SiOx /Si. This is attributed to a weakly passivating SEI and irreversible lithium trapping at the Si surface. Combinatorial screening of charge/discharge cycling by SECCM with co-located SIMS reveals SEI chemistry as a function of depth. While the SEI thickness is relatively independent of the cycle number, the chemistry - particularly in the intermediate layers - depends on the number of cycles, revealing the SEI to be dynamic during cycling. This work serves as a foundation for the use of correlative SECCM/SIMS as a powerful approach to gain fundamental insights on complex battery processes at the nano- and microscales.

A Shunt-Assisted Silicon Electrode for Micro Electrochemical Machining

Abstract Stray current causes undesired material dissolution in micro-electrochemical machining (micro-ECM). The reduction of stray corrosion, caused by stray current, continues to be a major challenge for accuracy improvement. To limit the distribution of stray current, a shunt-assisted silicon electrode, with an auxiliary anode sharing stray current, is proposed in this study. The auxiliary anode is arranged outside the insulating layer of the sidewall-insulated electrode. It is proved in simulation that the auxiliary anode can help reduce the average material removal rate on the machined surface by 55% and improve processing accuracy. A fabrication process of shunt-assisted silicon electrode by bulk silicon process and thin film deposition process are presented. Microgrooves and holes are machined in ECM experiments. The angle between each side-wall and the vertical plane is less than 10 deg. The gap between the sidewall of the machined structures and electrode-outer-contour is about 30 μm±6 μm for the grooves and 45 μm±10 μm for the holes. These long-term experiments and consistent processing results show the shunt-assisted electrode is reliable in ECM process. But due to the stray corrosion induced by DC power supply and conservative feed method, the effect of the shunt-assisted silicon electrode in inhibiting stray corrosion is not significant. In the future, a micro-ECM system with novel power supply and active control methodologies is expected to better utilize the effect of the shunt-assisted silicon electrode.

Atomic/Molecular Layer-Deposited Laminated Li2O-Lithicone Interfaces Enabling High-Performance Silicon Anodes.

Silicon-based materials are of long-standing interest as the anodes for next-generation lithium-ion batteries, yet their low initial Coulombic efficiency and poor interfacial stability are lethal limitations. In this work, we used atomic layer deposition (ALD) and molecular layer deposition (MLD) techniques to fabricate a lithium-containing laminated Li2O-lithicone hybrid film (∼5 nm) on a silicon electrode. The laminated film provides an additional surface Li source around silicon cores, which can partially reimburse the Li loss during battery cycling. Characterization of interfacial components shows that such a laminated Li2O-lithicone interface undergoes gentle element changes and participates in a hybrid solid electrolyte interphase with Li2CO3, Li2O, LixPOFy, and LiF species. Finite element model analysis and morphology characterization demonstrate that the laminated structure design can help relieve the interfacial stress and thus retain the integrity and reactivity of the silicon composite anode during cycling. Moreover, the lithium-based laminated film leads to a fast Li+ migration kinetics on the surface of the electrode as revealed by the galvanostatic intermittent titration technique and density functional theory calculation. Benefiting from the above merits, a silicon anode with a 91.2% initial Coulombic efficiency, a rate performance of 1460 mA h g-1 at 2 A g-1, and a reversible capacity over 646 mA h g-1 after 850 cycles was achieved. This work exemplifies the advantages of lithium-based hybrid films precisely engineered by ALD/MLD techniques for improving performances of advanced silicon anode batteries and deepens understandings on the mechanism of interfacial stability and reaction kinetics.

Tannic acid as a binder and electronic conductor precursor in silicon electrodes for Li-ion batteries

State of-the-art Si electrodes for Li-ion batteries typically require the addition of an advanced polymeric binder and a conductive carbon additive to the active material in order to circumvolve the issues associated with silicon, namely its poor conductivity in the oxidized state, and its high volume variation upon lithiation which give rise to several failure mechanisms. Here, we demonstrate that both additives can be replaced by a cheap, naturally available, polyphenol, tannic acid (TA). We show that Si/TA electrodes (80/20 wt%), even with a high loading (∼2 mg_Si cm−2), can retain a specific capacity above 2000 mAh g−1 after 50 cycles. This unexpected result appears to result from the irreversible reduction of TA during the first cycle, into a conducting, likely polymeric, species.

In/Ga-Doped Si as Anodes for Si-Air Batteries with Restrained Self-Corrosion and Surface Passivation: A First-Principles Study.

Silicon-air batteries (SABs) are attracting considerable attention owing to their high theoretical energy density and superior security. In this study, In and Ga were doped into Si electrodes to optimize the capability of Si-air batteries. Varieties of Si-In/SiO2 and Si-Ga/SiO2 atomic interfaces were built, and their properties were analyzed using density functional theory (DFT). The adsorption energies of the SiO2 passivation layer on In- and Ga-doped silicon electrodes were higher than those on pure Si electrodes. Mulliken population analysis revealed a change in the average number of charge transfers of oxygen atoms at the interface. Furthermore, the local device density of states (LDDOS) of the modified electrodes showed high strength in the interfacial region. Additionally, In and Ga as dopants introduced new energy levels in the Si/SiO2 interface according to the projected local density of states (PLDOS), thus reducing the band gap of the SiO2. Moreover, the I-V curves revealed that doping In and Ga into Si electrodes enhanced the current flow of interface devices. These findings provide a mechanistic explanation for improving the practical efficiency of silicon-air batteries through anode doping and provide insight into the design of Si-based anodes in air batteries.

Preparation of high specific surface mPSi/Al2O3/ZnO microcapacitors

In this study, a new concept is explored for an electrolyte-free microcapacitor fabrication based on mesoporous silicon (mPSi) and zinc oxide (ZnO) electrodes, separated by aluminum oxide (Al2O3) as a dielectric layer. For this conception, we gathered various techniques in order to have the best system, namely: •Electrochemical etching to form mPSi.•Aluminum (Al) thin film deposited on mPSi by physical vapor deposition (PVD).•Oxidation of Al in a rapid thermal processing (RTP) furnace heated to 600 °C under pure oxygen gas injection for 300 s.•Coating the ZnO layer on the assembly electrochemically. The designed device was characterized under DC and AC currents with a specific capacitance of 160 nF/cm2 for a DC of 40 nA/cm2, low series resistance (38.7 Ω) and high ohmic resistance (1.69 MΩ). The latest value of Rox indicates a very high ohmic resistance across the Al2O3 dielectric layer. The studied device shows good stability for galvanoplastic charge–discharge cycles exceeding 1200 cycles with an efficiency of about 60%. In AC, the decrease of the capacitance values with a frequency increase indicates the presence of numerous charge traps near the interface of Al2O3 with the electrodes. The conductance measurements demonstrate a high interface state density (Nss) value of 1.6 × 1012 cm−2. The calculated leakage current was about two to three times higher than that obtained in the literature.

Topology Optimization with Matlab: Geometrically Non-Linear Optimum Solid Structures at Random Force Strengths

This paper aims to investigate multiple large-strain topology-optimized structures, by interpreting their overlay as a probability density function. Such a strategy is suited to finding an optimum design of silicon electrodes subject to a random contact. Using this method, and prescribing a zero net-force constraint on the global system, the optimum structure is identified with a Schwarz P minimum-surface structure. Then, the optimum structure is subject to chemo-mechanically coupled cycling, in terms of an irreversible thermodynamic process, which shows the interplay between the mechanical and chemical fields. The Matlab-based optimization code is attached.

Constructing a conductive and buffer network on microscale silicon-based anodes for high-performance lithium-ion batteries

The structure and characteristics of the electrodes are essential for the performance of microscale silicon-based anodes for lithium-ion batteries. In this study, various carbon sources with different degrees of graphitization, morphologies, and dispersities were utilized as conductive agents for a microstructured silicon electrode. The findings indicate that micron-sized silicon electrodes can benefit from the addition of flake-conductive graphite, particularly SFG-6, which possesses a high degree of graphitization and dispersion, as well as a particle size similar to that of silicon. This combination results in a well-distributed, uniform conductive and buffering network, leading to improved electrochemical performance overall. After 450 cycles, the Si-SFG-6 composite anode exhibited exceptional long-term stability, delivering a specific capacity of 1102 mA·h g−1 at a current density of 200 mA g−1. Furthermore, even at a higher current density of 2000 mA g−1, the reversible capacity remained impressive at 964 mA·h g−1. The results of this study offer valuable insights for optimizing the structure and properties of microstructured silicon-based anodes, with the aim of achieving superior performance in Li-ion batteries.

The Effects of Silicon Anode Thickness on the Electrochemical Performance of Li-Ion Batteries

The electrode configuration is an important element in the development of Li-ion cells. The energy density is proportional to the loading of the active material. Therefore, increasing the electrode thickness is the simplest way to achieve higher capacities. In this paper, we compare the effects of three different thicknesses of Ag-decorated Si electrode anode (HCSi) on the electrochemical performances such as the SEI layer formation, impedances, and mass capacitances. We prepared three different silicon electrode thicknesses to optimize the electrodes: 20, 40 and 60 µm and measured in situ galvanostatic electrochemical impedance spectroscopy (GEIS). Using GEIS, we studied the intercalation mechanism of Li+ ions in detail and found that despite having the same capacities (≈3500 mAh g−1), the thinnest electrode, HCSi20, allows diffusion of Li+ ions into the bulk, whereas thicker layers prevent smooth diffusion into the bulk of the silicon electrode due to increased layer resistance. The Voigt model was used to analyze the anomaly of the frequency dependence of the measured impedance, in which, the classical Randles circuit is connected in series with one or two R ‖ C parallel combinations. One R ‖ C circuit could be the result of the SEI formation, and the second R ‖ C circuit could be the contribution of Li. To increase the number of charge and discharge cycles, we improved the electrolyte by adding fluoroethylene carbonate (FEC), which reduced the capacity of the HCSi20 electrode to 50% of the initial capacity (≈3500 mAh g−1) after 60 cycles, whereas it dropped to 20% of the initial capacity after 10 cycles without the addition of FEC.

A multiplicative finite strain deformation for diffusion-induced stress: An incremental approach

Understanding the evolution of lithium concentration and stresses in electrode materials during electrochemical cycling is of paramount importance in studying the fundamental mechanisms contributing to the structural degradation of lithiated electrode materials. In this work, we use the change rate of the volume of a host material in analyzing diffusion-induced volume expansion and obtain the volume expansion as an exponential function of the concentration of solute atoms and the coefficient of linear expansion in contrast to the widely used linear relation between the diffusion-induced volume expansion and the coefficient of linear expansion. Using the principle of multiplicative decomposition in the framework of elastic deformation and thermodynamic free energies, we derive the constitutive relation for diffusion-induced finite deformation of elastic materials. The symmetric Piola-Kirchhoff stress is an exponential function of the coefficient of linear expansion and the concentration of solute atoms. Incorporating the change of strain field with the change in the number of solute atoms in Helmholtz free energy, we derive the chemical potential of the solute atoms in the host material the same as the one given by Li et al. (Zeitschrift für PhysikalischeChemie, 49 (1966) 271–290). Numerical results for the lithiation of a spherical silicon electrode reveal that the spatial distributions of lithium concentration and stresses for small dimensionless time (dilute solution) exhibit similar trends to the corresponding ones calculated from the theory of linear elasticity and nonlinear characteristics for large dimensionless time (concentrated solution).

Link between anisotropic electrochemistry and surface transformations at single‐crystal silicon electrodes: Implications for lithium‐ion batteries

AbstractSilicon is a promising negative electrode material for high‐energy‐density Li‐ion batteries (LiBs) but suffers from significant degradation due to the mechanical stress induced by lithiation. Volume expansion and lithiation in Si are strongly anisotropic but associated early interfacial transformations linked to these phenomena and their implications for electrode performance remain poorly understood. Here we develop a novel correlative electrochemical multi‐microscopy approach to study local interfacial degradation at the early stages for three different surface orientations of Si single crystals: Si(1 0 0), Si(1 1 0) and Si(3 1 1), after Li‐ion electrochemical cycling. The experimental strategy combines scanning electrochemical cell microscopy (SECCM) measurements with subsequently recorded scanning transmission electron microscopy images of high‐quality cross sections of Si electrodes, extracted at selected SECCM regions, using a novel Xe+ plasma‐focused ion beam procedure. These studies reveal significant surface orientation–dependent nanoscale degradation mechanisms that strongly control electrode performance. Si(1 0 0) was immune to interfacial degradation showing the best lithiation reversibility, whereas local nanoscale delamination was observed in Si(1 1 0) leading to a lower Coulombic efficiency. Continuous electrochemical deactivation of Si(3 1 1) was associated with delamination across the whole interface, Li trapping and formation of thick (ca. 60 nm) SiO2 structures. These results demonstrate surface crystallography to be a critical factor when designing Si‐based battery materials and strongly suggest that promoting Si(1 0 0) facets could potentially provide longer cycling life and performance due to a higher resistance to degradation.

Density Functional Tight-Binding Model for Lithium-Silicon Alloys.

The predictive power of molecular dynamic simulations is mainly restricted by the time scale and model accuracy. Many systems of current relevance are of such complexity that they require addressing both issues simultaneously. This is the case of silicon electrodes in Li-ion batteries, where different LixSi alloys are formed during charge/discharge cycles. While first-principles treatments for this system are seriously limited by the computational cost of exploring its large conformational space, classical force fields are not transferable enough to represent it accurately. Density Functional Tight Binding (DFTB) is an intermediate complexity approach capable of capturing the electronic nature of different environments with a relatively low computational cost. In this work, we present a new set of DFTB parameters suited to model amorphous LixSi alloys. LixSi is the usual finding upon cycling the Si electrodes in the presence of Li ions. The model parameters are constructed with a particular emphasis on their transferability for the entire LixSi composition range. This is achieved by introducing a new optimization procedure that weights stoichiometries differently to improve the prediction of their formation energies. The resulting model is shown to be robust for predicting crystal and amorphous structures for the different compositions, giving excellent agreement with DFT calculations and outperforming state-of-the-art ReaxFF potentials.