Lithiation Of Silicon Research Articles

Single Particle Measurement for Volume Change Behavior of a Silicon Single Particle

Lithium-ion batteries (LIBs) with high gravimetric and volumetric energy density are very important key devices for the establishment of the sustainable energy system which is consisted of solar cells, wind power generations, smart grid, and batteries. In addition, the LIBs are extensively expected for a power supply of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Much higher energy density is needed to increase the mileage per charge. Current LIBs use the composite electrodes which are composed of active materials, binders, conductive agents, and current collector. The charging and discharging performance of the composite electrode must be affected by the “composite” state (kinds of materials, mixing ratio, thickness, and tap density). In addition to that, the materials which have large volume change during the charging and discharging are strongly affected by the state of the composite electrode. It is difficult to distinguish the electrochemical characteristics of only active materials from that of the composite electrode.However, the intrinsic properties of active materials are very important to evaluate the electrode performance and understand the mechanism of the charging and discharging reaction. In this study, the single particle measurement technique was used to study the electrochemical lithiation of silicon single particle. This technique is very useful to measure the intrinsic properties of single particle of the active materials in the actual liquid electrolyte. Our group has reported that some positive electrode active materials have very good rate characteristics [1,2]. The microelectrode with 20 mm diameter is used as the micro probe to contact and apply the electrochemical technique to the one active material particle. The electrodeposited Cu covers the tip of the micro probe because the Cu is stable against the electrode potential of the lithiation of silicon. Our previous study has reported that this technique is applicable to measure the volume expansion of silicon single particle during the lithiation [3]. Figure 1 shows the drastic volume expansion behavior of a silicon single particle during the lithiation. A silicon particle contacted with micro-probe was expanding with the lithiation. The apparent volume expansion ratio is larger than the theoretical expectation. So far, we estimate that the reasons are the aggregation state of the silicon particle, the emergence of amorphous phase of the lithiated silicon and anisotropic properties of volume change. In order to discuss the reason of larger volume expansion and volume change mechanism of the silicon particle more precisely, micro-tweezers system was used to move the silicon particle to SEM observation and Raman spectroscopy measurement after the 1st charging by the single particle measurement technique. Some researchers use an in-situ TEM technique in order to understand the mechanism of silicon lithiation and delithiation [4]. This excellent technique can observe the silicon volume change behavior accompanied with the lithiation and delithiation with very high magnification, but it needs high vacuum condition. The advantage of our technique is the electrochemical condition is very near to the actual LIBs. Therefore, it is important to combine their knowledge and our results to find the volume change mechanism of silicon electrode during the lithiation and delithiation. In this presentation, we would like to discuss the comparison.Furthermore, high capacity active materials which have volume change during charging and discharging need the way to restrict or accommodate the volume change for the usage to practical battery electrodes. From the viewpoint of such approach, the binder effect is very important. A small piece of composite electrode with 20~40 mm was picked up to measure the electrochemical characteristics in the single particle measurement system. Comparing with the pristine silicon particle measurement results, we would like to discuss the binder effect to volume change behavior. References (1) K. Dokko, N. Nakata, K. Kanamura, J. Power Sources 189, 783 (2009)(2) H. Munakata, B. Takemura, T. Saito, K. Kanamura, J. Power Sources 217, 444 (2012)(3) K. Nishikawa, H. Munakata, K. Kanamura, J. Power Sources 243,630 (2013) [4]. M. T. McDowell, S. W. Lee, J. T. Harris, B. A. Korgel, C. Wang, W. D. Nix, and Y. Cui, Nano Lett., 13, 758 (2013)Figure 1. Volume expansion behavior of silicon single particle during the first lithiation

A chemo-mechanical model of lithiation in silicon

We present a chemo-mechanical model to investigate the lithiation-induced phase transformation, morphological evolution, stress generation and fracture in crystalline silicon nanowires (c-SiNWs). The model couples lithium (Li) diffusion with elasto-plastic deformation in a three-dimensional (3D) setting. Several key features observed from recent transmission electron microscopy (TEM) studies are incorporated into the model, including the sharp interface between the lithiated amorphous shell and unlithiated crystalline core, crystallographic orientation dependent Li–Si reaction rate, and large-strain plasticity. Our simulation results demonstrate that the model faithfully predicts the anisotropic swelling of lithiated SiNWs observed from previous experimental studies. Stress analysis from the finite-deformation model reveals that the SiNWs are prone to surface fracture at the angular sites where two adjacent {110} facets intersect, consistent with previous experimental observations. The mechanistic understanding of the morphological evolution and stress generation sheds light on the design of failure-resistant nanostructured electrodes. Our model offers a framework for the study of the chemo-mechanical degradation in high-capacity electrode materials.

Nanostructured Porous Silicon - Carbon Anodes for Lithium-Ion Batteries: Two Novel Approaches

Silicon-based electrodes for Li-ion batteries (LIB) have recently attracted much attention because of their high theoretical capacity (~4000 mAh/g) and low average voltage (0.2 V) versus Li. However, the large volume change (~300%) due to Li-Si alloy formation results in poor cycling performance due to mechanical cracking and pulverization. It is believed that nanostructuring can help alleviate the effects of volume change and lead to improved cycling behaviour.In this work, two novel approaches for composite Si-C anodes for lithium ion microbatteries are presented, based on the deposition of nanostructured porous Si films by Pulsed Laser Deposition (PLD) at room temperature and its coupling to a C layer.In detail, the one approach is based on covering the as-deposited PLD porous Si films by a disordered graphitic C layer deposited by CVD that promotes the formation of a stable solid electrolyte interphase (SEI) layer, which is a critical step for an effective use of Si in LIB. In addition, the annealing effect induced by the CVD process on the anode is thought to be beneficial for its stability.In the second approach, nanostructured Si is deposited by PLD on a foil made of carbon nanotubes (CNTs) that acts also as a current collector, thus making the underlying standard copper collector unnecessary. This dramatically reduces the thickness of the overall electrode, thus addressing the requirements for application in innovative microbatteries.Introducing voids at the nanoscale in the Si film by nanostructuring is meant to accommodate the large volume expansion associated with silicon lithiation with no losses in film continuity (in contrast, for example, with films made of Si nanoparticles), while C is well known to be the standard material for LIB anodes for its good cycle life and capability of forming stable SEI layer with commonly used electrolytes. Moreover, the as-deposited Si film exhibits an amorphous structure in high-resolution TEM (HRTEM) and this avoids its amorphization during the first cycles, turning into lower internal mechanical stresses. Structure and morphology of the C-coated porous silicon samples have been widely investigated by suitable microscopic and spectroscopic techniques and their electrochemical behaviour characterized for different current densities.With CVD carbon coating, porous silicon promisingly performs in a very stable way, showing areal capacity of ~175µAh/cm2 at 54 µA/cm2, and no decay for at least 1000 cycles.On the other hand, also porous nanostructured silicon deposited on CNTs performs promisingly, showing areal capacity of ~150µAh/cm2 at 1080µA/cm2 with no decay for at 1000 cycles.

Mesoporus Silicon through Magnesium Reduction of Polymer Templated Silica for High Power Li-Ion Batteries

This work describes the synthesis and electrochemical characterization of mesoporous silicon for high power Li-ion battery applications. Silicon is an attractive Li-ion battery anode material for its high energy density, high elemental abundance and non-toxicity. However, full lithiation of silicon is accompanied by a 300% volume expansion which pulverizes the particles leading to rapid material failure. We have addressed these issues with silicon by developing a highly tunable synthesis method to produce mesoporous silicon from ordered mesoporous silica. This synthesis method leads to micron sized grains consisting of a 3-D interconnected silicon network that should accommodate the electrochemical induced volume expansion while the nanocystalline pore walls provide short ion diffusion lengths for high rate applications.The ordered mesoporous silica starting material is synthesized by evaporation induced self-assembly of sol-gel type precursors and a di-block copolymer. The mesoporous silicon is produced through reduction of the silica precursor with magnesium metal at 650 °C followed by selective chemical etching. The particle morphology and the pore size of both the silica and the silicon can be tuned over a broad range, and have been characterized by small angle scattering, high angle X-ray diffraction, nitrogen porosimetry, and SEM imaging. This synthesis technique produces 1-10 µm silica particles with a cubic lattice of 15 – 20 nm pores and 10 - 15 nm thick pore walls. After magnesium reduction, the porous polycrystalline silicon retains the microscale morphology and the porous nanostructure of the parent material. The silicon pore walls consist of ~30 nm crystallites which are optimal for high power density electrochemical applications.Magnesium reduction of silica is an effective method to reduce silica while preserving the nanostructured architecture, but little is understood about reaction mechanism. In an effort to control the nanoscale architecture of silicon we have investigated the reaction mechanism. We have performed in-situ X-ray diffraction during the reduction process combined with Raman spectroscopy to reveal a multi-step reaction pathway which leads to the mesoporous crystalline silicon final product. This study demonstrates that the reaction of mesoporous silica and magnesium metal first forms amorphous silicon that further reacts to form crystalline magnesium silicide. The remaining mesoporous silica is reduced by both the magnesium metal and the magnesium silicide to produce mesoporous crystalline silicon.The electrochemical properties of mesoporous silicon have been characterized by galvanostatic cycling and cyclic voltammetry. Galvanostatic measurements show capacities on the order of 1700 mAh/g while the nanocrystalline silicon pore walls and interconnected 3-D framework led to an extremely high rate capability. Over 40% of the capacity, or 720 mAh/g, can be accessed in 150 seconds. In light of the high capacity and highest reported power density for mesoporous silicon, this material is an exciting candidate for the next generation of high performance Li-ion batteries.

Early Identification of the Li15Si4 Phase Using in-Situ Synchrotron XRD

In-situ synchrotron X-ray powder diffraction (SXRD) was performed on a Li-Si pouch cell on the high-energy beamline (I12) at Diamond Light Source. The high photon energies provided by the synchrotron allowed the beam to pass directly through the Li-Si pouch cell while still providing high resolution SXRD data. Furthermore, the large flux of the synchrotron beam allows high temporal resolution SXRD data to be collected. Fast acquisition time and high resolution SXRD patterns allows for accurate identification of the onset of Li15Si4 formation.Obrovac and Christensen [1] showed that the fully lithiated phase for silicon at room temperature is Li15Si4. They also showed that the Li15Si4 phase is formed below 50 mV (vs. Li), which typically occurs at the end of silicon lithiation. Li and Dahn [2] used in-situ XRD to show that the Li15Si4 phase does not occur until high capacity (> 3500 mAh/g) and low voltage (< 50 mV). The results discussed above clearly show that formation of the Li15Si4 phase can be capacity dependent. One could also theorize that the formation of Li15Si4 is voltage dependent. To further elucidate the effect of voltage on the formation of the Li15Si4 phase in-situ SXRD was performed on Li-Si cells at voltages below 50 mV. In-situ SXRD data is shown in Figure 1 as a function of intensity (left ordinate) and as a ‘waterfall plot’ vs. time (right ordinate).Figure 1 clearly shows that the silicon peak recedes as a function of time (and capacity), while the Li15Si4peak grows. To better quantify the formation of the Li15Si4 phase Figure 2 shows the evolution of the volume fraction of Li15Si4 as a function of time (lower plot abscissa) and capacity (upper plot ordinate).Figure 2 shows that the Li15Si4 phase can be observed almost immediately. As capacity increases the volume fraction of Li15Si4 continues to increase. These results show that the Li15Si4phase can be formed very early if the voltage at the silicon electrode is below 50 mV.

In Situ Observation of Epitaxial Li-Si-Nanostructure Formation on Si(111)

Silicon is considered as anode material for Li-ion batteries due to its high theoretical capacity with a value of 4200 mAh/g for Li22Si5. The associated volume change exceeding 300 % during de-/lithiation of silicon is considered as the major problem for application with a possible solution in using nanostructured material. Various details of the formation of Li-Si phases are presently under discussion like the formation of amorphous vs. crystalline structures or the Li-Si phase order during lithiation.To shed light on the relevant processes during the Li-Si phase formation a clean Si substrate was exposed under ultra-high condition in a molecular beam epitaxy (MBE) environment to an atomic lithium flux. During the experiment the Si substrate temperature was gradually reduced from initial 700°C down to room temperature. Simultaneously, the onset of the nucleation and the formation of Li-Si phases were observed by in situ synchrotron x-ray diffraction (XRD), azimuthal reflection high-energy electron diffraction (ARHEED), and line-of-sight quadrupole mass spectrometry (QMS) of the desorbing Li. Initial only Si surface reconstructions are observed involving only the top most atomic layer. At lower temperatures, Li-Si nanostructures are formed. First, crystalline Li12Si7 appears, followed by the formation of Li21Si5 phases. Both phases grow epitaxially. The epitaxial relationship is Si[111]||Li12Si7[010] and Si[11-2]||Li12Si7[100] as well as Si[111]||Li21Si5[111] and Si[01-1] ||Li21Si5[01-1]. Further confirmation besides the XRD data is gained by investigating the Li-Si surfaces. The Li21Si5(111) surface exhibits an approximately nine times larger (1x1) surface unit cell than the unreconstructed Si(111) surface. Ex situ investigations of various intermediate stages of the Li-Si phase formation by atomic force microscopy show nanostructures which formed on the substrate surface with a height of about 10 nm and 100 nm diameter much smaller than usually reported in Si anode material. It demonstrates that the formation of the Li-Si phases depends on the Li kinetics as well as the Si release mechanism from the crystalline substrate and its diffusion. Acknowledgements: This work was supported by the Leibniz-Gemeinschaft under the project No. SAW-2011-PDI-230. The experiments are carried out at the synchrotron radiation facility Bessy II (Helmholtz-Zentrum Berlin, Germany).

In Situ Infrared Studies at the Semiconductor/Electrolyte Interface and Application to Lithium-Ion Batteries

In-situ spectroscopic characterization of electrode/ electrolyte interfaces is of paramount importance in order to complement classical electrochemical investigation tools and obtain a full understanding of the investigated electrochemical systems. This approach has been shown to be quite useful for semiconductor electrodes, since the chemistry of the semiconductor surface often plays a dominant role in the behavior of the whole system.At the semiconductor/electrolyte interface, the multiple-reflection, attenuated-total-reflection (ATR) geometry has long proved to be the most useful experimental tool for maximizing the surface vibrational signals and minimizing the electrolyte absorption [1]. This advantage is gained without suffering from the mass-transport limitations inherent to the thin-electrolyte-layer spectroelectrochemical cells needed for external reflection geometry. Using polarized infrared beams in ATR geometry also allows for quantitative measurements to be performed and structural information to be extracted. When coupled to a modulation technique, dynamic information can also be accessed.Many applications of the technique have been demonstrated, especially for the vibrational characterization of interface and double-layer species under electrochemical control (e.g., surface chemical changes, changes in the adsorption of neutral or ionic species, or changes in the ionic composition of the double layer as a function of the applied potential). Changes in the concentration of IR absorbing species outside the double layer but close to the surface can also be detected (e.g., measurement of the local pH). More specific information can be obtained on the semiconductor side such as changes in the electronic absorption (from electronic surface states or from free carriers) upon external stimuli or electrochemical insertion of specific species (e.g., semiconductor hydrogen loading under cathodic conditions). Finally, the technique has been proved very useful for the study of electrochemically grown thin films (e.g., for the detailed characterization of anodic films). Specific cells allow for performing such studies in aqueous or anhydrous solvents, and under potentiostatic or galvanostatic control, hydrodynamic control, temperature control, or illumination.Therefore, in situ infrared spectroscopy appears as a unique technique to investigate electrode surface properties in lithium-ion batteries. Moreover, the advantages brought by using multiple-reflection, ATR geometry allow for overcoming the limitations encountered in the other in-situ IR studies of these systems. Silicon electrodes (which represent an impressive gain in energy density for negative electrodes in Li-ion batteries) have therefore been studied in a half-cell configuration against a lithium-metal electrode. The use of a thin-film geometry prevents electrode mechanical failure when films are thin enough and provides a well-controlled geometry allowing for the quantitative measurements of electrode evolutions, e.g., the formation of the solid-electrolyte interphase (SEI). Electrodes are thin films of hydrogenated amorphous Si deposited on a crystalline Si ATR prism, and experiments have been performed in 1M LiClO4 in PC or in 1M LiPF6in PC:EC:3DMC (2% VC, 10% FEC) electrolyte. As shown in Fig. 1, experimental spectra consist of positive peaks characteristic of the SEI film formed during the electrochemical process, and negative peaks due to electrolyte exclusion from the interface due to the SEI growth. The latter peaks can be used for quantitatively monitoring the film growth, which allows for determining the evolution of the SEI thickness during the first lithiation and after subsequent cycles (Fig. 2). A noticeable effect is the SEI growth and partial disappearance during the first lithiation/delithiation cycle. The experimental spectra can also be corrected for electrolyte absorption, which allows for isolating the SEI spectra and characterize its composition. In this way, Li solvation effects can also be evidenced close to the SEI. Additional information includes effects of lithiation on the bulk silicon thin-film, where the Si-H bonds, present in abundance in the a-Si:H network, are gradually and irreversibly consumed along silicon lithiation. Electronic absorption also evidences a distinct lithiation process in the first cycle (progressive invasion of a heavily lithiated phase from the silicon outer surface) as compared to subsequent cycles (quasi-homogeneous lithiation).[1] J.-N. Chazalviel, B. H. Erné, F. Maroun, F. Ozanam, J. Electroanal. Chem. 509 (2001) 108-178.

In Situ Atomic Force Microscopy Studies of Silicon Lithiation and Delithiation in Liquid Electrolytes

Understanding the chemical and mechanical response of silicon (Si) to lithiation and delithiation in liquid, organic electrolytes is critical to successfully introducing Si into rechargeable lithium ion batteries. Among Si materials, nanoscale Si, especially amorphous Si (a-Si), has demonstrated some of the best performance to date. Nevertheless, an understanding of solid electrolyte interphase (SEI) growth and volume change hysteresis during cycling is limited. To this end, in situ experiments are conducted using an atomic force microscope (AFM) to acquire both qualitative and quantitative data. Using several electrolytes and electrolyte additives, the growth of SEI on thin film Si is analyzed. Additionally, precision Si nanostructures are fabricated using electron beam lithography, and the SEI growth, volume changes, and mechanical properties of these structures are recorded. The mechanical properties including elastic modulus and hardness are determined by indenting the nanopillar with a well-characterized AFM tip such that images of the pillar are acquired along with the numerical data of the indent. The results demonstrate the Si suffers permanent degradation of mechanical properties on the first cycle despite appearing to physically remain intact. The ability to determine the structural integrity of the Si before, during, and after lithiation in conventional, liquid electrolytes with and without additives is unique to AFM techniques which physically probe the sample. Additionally, in situ AFM studies of nanoporous Si and nanoscale Si structures covered by atomic layer deposition (ALD) thin films are discussed. The in situ AFM results presented in this work will help to guide experimental and computational modeling research by providing both qualitative information on how SEI on Si grows as well as numerical values for mechanical properties of Si as it is lithiated and delithiated.

Lithium Intercalation Behavior in Multilayer Silicon Electrodes

Next generation lithium battery materials will require a fundamental shift from those based on intercalation to elements or compounds that alloy directly with lithium. Intermetallics, for instance, can electrochemically alloy to Li4.4M (M = Si, Ge, Sn, etc.), providing order‐of‐magnitude increases in energy density. Unlike the stable crystal structure of intercalation materials, intermetallic‐based electrodes undergo dramatic volume changes that rapidly degrade the performance of the battery. Here, the energy density of silicon is combined with the structural reversibility of an intercalation material using a silicon/metal‐silicide multilayer. In operando X‐ray reflectivity confirms the multilayer's structural reversibility during lithium insertion and extraction, despite an overall 3.3‐fold vertical expansion. The multilayer electrodes also show enhanced long‐term cyclability and rate capabilities relative to a comparable silicon thin film electrode. This intercalation behavior found by dimensionally constraining silicon's lithiation promises applicability to a wide range of conversion reactions.

Investigation of Initial Lithiation of Silicon (100) Using Solid-State 7Li NMR

Structural changes in amorphous LixSi during the initial lithiation of single crystal Si to amorphous LixSi were investigated as a function of Li flux and total charge using solid-state 7Li nuclear magnetic resonance (NMR) spectroscopy. Single crystal (100) Si wafers were used as a model system in this study, as the Li flux could be precisely controlled and uniformly distributed across the surface of the wafer. It was observed that peak shifts in solid-state 7Li NMR spectra varied as a function of the Li flux during the initial lithiation and stayed constant for samples of the same Li flux regardless of the total charge applied to the electrodes. We conclude from these results that the Li concentration in LixSi stays constant for a given flux regardless of the total coulombic charge applied to the electrode. The results from this study help better understand the kinetics, the reaction mechanisms, and the kinetic modeling of chemical reactions at the reaction front during the initial lithiation of Si (100).

Orbital-free density functional theory study of crystalline Li–Si alloys

Li–Si interactions are of great interest currently due to the potential use of silicon anodes in Li-ion batteries. As a first step toward eventual nanoscale characterization of lithiation of silicon, here we study the crystalline Li–Si alloys LiSi, Li12Si7, Li7Si3, Li13Si4, Li15Si4, and Li22Si5 using orbital-free density functional theory (OFDFT). The recently proposed Wang–Govind–Carter decomposition (WGCD) and Huang–Carter (HC) kinetic energy density functionals (KEDFs) are used to evaluate the electron kinetic energy. Both KEDFs predict accurate cell lattice vectors, equilibrium volumes, bulk moduli, and ground-state densities when compared to Kohn–Sham density functional theory (KSDFT) benchmarks. Elastic constants and alloy formation energies calculated with the WGCD KEDF also agree reasonably well with KSDFT. Finally, Li atom adsorption energies on the Si(100) − 2 × 1 surface are calculated as a simple initial test of the Li–Si mixing process during lithiation of silicon. The OFDFT adsorption energies again are fairly close to KSDFT values. The results in this work demonstrate the accuracy of the WGCD and HC KEDFs for materials with mixed covalent-metallic character and their considerable transferability under different chemical environments. Because of its quasilinear scaling, coupled with the level of accuracy shown here, OFDFT appears quite promising for large-scale simulation of such materials phenomena.

ChemInform Abstract: 25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium‐Ion Batteries

AbstractReview: 145 refs.

25th Anniversary Article: Understanding the Lithiation of Silicon and Other Alloying Anodes for Lithium‐Ion Batteries

Alloying anodes such as silicon are promising electrode materials for next-generation high energy density lithium-ion batteries because of their ability to reversibly incorporate a high concentration of Li atoms. However, alloying anodes usually exhibit a short cycle life due to the extreme volumetric and structural changes that occur during lithium insertion/extraction; these transformations cause mechanical fracture and exacerbate side reactions. To solve these problems, there has recently been significant attention devoted to creating silicon nanostructures that can accommodate the lithiation-induced strain and thus exhibit high Coulombic efficiency and long cycle life. In parallel, many experiments and simulations have been conducted in an effort to understand the details of volumetric expansion, fracture, mechanical stress evolution, and structural changes in silicon nanostructures. The fundamental materials knowledge gained from these studies has provided guidance for designing optimized Si electrode structures and has also shed light on the factors that control large-volume change solid-state reactions. In this paper, we review various fundamental studies that have been conducted to understand structural and volumetric changes, stress evolution, mechanical properties, and fracture behavior of nanostructured Si anodes for lithium-ion batteries and compare the reaction process of Si to other novel anode materials.

Lithiation of silicon via lithium Zintl-defect complexes from first principles

An extensive search for low-energy lithium defects in crystalline silicon using density-functional-theory methods and the ab initio random structure searching (AIRSS) method shows that the four-lithium-atom substitutional point defect is exceptionally stable. This defect consists of four lithium atoms with strong ionic bonds to the four under-coordinated atoms of a silicon vacancy defect, similar to the bonding of metal ions in Zintl phases. This complex is stable over a range of silicon environments, indicating that it may aid amorphization of crystalline silicon and form upon delithiation of the silicon anode of a Li-ion rechargeable battery.

Study of lithiation mechanisms in silicon electrodes by Auger Electron Spectroscopy

Despite many studies, silicon lithiation mechanisms remain unclear. Si amorphization induced by lithium insertion hinders the use of standard techniques of characterization such as diffraction. In this paper, electrochemical silicon lithiation is investigated by using a technique of surface characterization, Auger Electron Spectroscopy (AES). The first part of this work has been dedicated to evaluating the possibility of studying Li–Si alloys with AES: well-known crystalline LixSi alloys were successfully synthesized and analyzed. Based on the Auger results, Alloy Reference Relative Sensitivity Factors (ARRSFs) were determined for each alloy. After this step, several Si-electrodes were scrutinized at different Depths Of Discharge (DOD) during the first lithiation. The very high spatial resolution of AES allowed us to investigate individual silicon particles with respect to their electrode environment and study the lithiation mechanisms which occur at the particle scale. The core–shell model is confirmed: Si lithiation starts with the formation of a Li-rich shell surrounding a core of pure silicon. By using the ARRSFs calculated previously, an accurate composition of this shell can be determined: Li3.1Si. This phase grows upon lithiation resulting in a progression of the two-phase boundary inside the particle at least until a discharge capacity of 1800 mA h g−1. AES appears then as a very valuable technique to better understand the lithium insertion process in silicon as well as the failure mechanisms of Si-electrodes and can help in the development of high performance Si materials.

Anisotropic lithiation behavior of crystalline silicon

In this paper we demonstrate the anisotropic lithiation of silicon (Si). Therefore specimens with radiating Si beams were selectively covered with pure lithium (Li) and lithiation was investigated at different temperatures. Due to the radial arrangement, Si beams underwent a crystallographic orientation dependent lithiation. The experiments showed up to 40% faster lithiation in 〈110〉 crystallographic directions compared to 〈100〉 oriented Si, and a temperature dependence of the lithiation propagation with lithiation rates of up to 337 nm s−1 at 100 °C. These results were reflected in prior observation of 〈110〉-orientated Li–Si dendrites, formed after mechanical contact of Li with a Si device layer of a silicon-on-insulator wafer.

Interface-reaction controlled diffusion in binary solids with applications to lithiation of silicon in lithium-ion batteries

Solid state diffusion in a binary system, such as lithiation into crystalline silicon, often involves two symbiotic processes, namely, interfacial chemical reaction and bulk diffusion. Building upon our earlier work (Cui et al., 2012b, J. Mech. Phys. Solids, 60 (7), 1280–1295), we develop a mathematical framework in this study to investigate the interaction between bulk diffusion and interfacial chemical reaction in binary systems. The new model accounts for finite deformation kinematics and stress–diffusion interaction. It is applicable to arbitrary shape of the phase interface. As an example, the model is used to study the lithiation of a spherical silicon particle. It is found that a dimensionless parameter β=kfeVmBR0/D0 plays a significant role in determining the kinetics of the lithiation process. This parameter, analogous to the Biot number in heat transfer, represents the ratio of the rate of interfacial chemical reaction and the rate of bulk diffusion. Smaller β means slower interfacial reaction, which would result in higher and more uniform concentration of lithium in the lithiated region. Furthermore, for a given β, the lithiation process is always controlled by the interfacial chemical reaction initially, until sufficient silicon has been lithiated so that the diffusion distance for lithium reaches a threshold value, beyond which bulk diffusion becomes the slower process and controls the overall lithiation kinetics.

Inexpensive method for producing macroporous silicon particulates (MPSPs) with pyrolyzed polyacrylonitrile for lithium ion batteries

One of the most exciting areas in lithium ion batteries is engineering structured silicon anodes. These new materials promise to lead the next generation of batteries with significantly higher reversible charge capacity than current technologies. One drawback of these materials is that their production involves costly processing steps, limiting their application in commercial lithium ion batteries. In this report we present an inexpensive method for synthesizing macroporous silicon particulates (MPSPs). After being mixed with polyacrylonitrile (PAN) and pyrolyzed, MPSPs can alloy with lithium, resulting in capacities of 1000 mAhg−1 for over 600+ cycles. These sponge-like MPSPs with pyrolyzed PAN (PPAN) can accommodate the large volume expansion associated with silicon lithiation. This performance combined with low cost processing yields a competitive anode material that will have an immediate and direct application in lithium ion batteries.

In situ atomic-scale imaging of electrochemical lithiation in silicon

In lithium-ion batteries, the electrochemical reaction between the electrodes and lithium is a critical process that controls the capacity, cyclability and reliability of the battery. Despite intensive study, the atomistic mechanism of the electrochemical reactions occurring in these solid-state electrodes remains unclear. Here, we show that in situ transmission electron microscopy can be used to study the dynamic lithiation process of single-crystal silicon with atomic resolution. We observe a sharp interface (~1 nm thick) between the crystalline silicon and an amorphous Li(x)Si alloy. The lithiation kinetics are controlled by the migration of the interface, which occurs through a ledge mechanism involving the lateral movement of ledges on the close-packed {111} atomic planes. Such ledge flow processes produce the amorphous Li(x)Si alloy through layer-by-layer peeling of the {111} atomic facets, resulting in the orientation-dependent mobility of the interfaces.

Understanding the Interfacial Processes at Silicon–Copper Electrodes in Ionic Liquid Battery Electrolyte

The interfacial processes at an amorphous silicon−copper (Si−Cu) electrode in the ionic liquid electrolyte of 1 M lithium bis(trifluoromethanesulfonyl)imide/1-methyl-1-propylpyrrolidinium bis- (trifluoromethylsulfonyl)imide (LiTFSI/Py1,3TFSI) during initial charge− discharge cycle are studied by characterizing the solid electrolyte interphase (SEI) composition at different states of charge using ex situ attenuated total reflection FTIR spectroscopy combined with X-ray photoelectron spectroscopy. The analyses data reveal that in the very early stage of charge (1.5 V vs Li/Li + ), alkylated Si and ester-containing species first form by the reductive decomposition of Py1,3 ion, and LiF salt and Si−F bond- containing compound first form by the decomposition of TFSI anion, respectively. TFSI decomposition is observed to begin with the C−F cleavage, which was proposed as the cleavage of N−S bond in the previous reports. Charging to lower voltage thickens the SEI layer, but lithiation of silicon results in damage or destabilization of the existing SEI probably due to changes in structural volume together with particle morphology. The SEI layer is however reversibly rebuilt in the course of delithiation. The data provide a basic understanding of the SEI formation mechanism on the silicon-based anodes in ionic liquid electrolyte for less flammable batteries.