Investigating the role of crystallographic orientation of single crystalline silicon on their electrochemical lithiation behavior: Surface chemistry of Si determines the bulk lithiation (original) (raw)
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Journal of the American Chemical Society, 2012
Silicon is of significant interest as a next-generation anode material for lithium-ion batteries due to its extremely high capacity. The reaction of lithium with crystalline silicon is known to present a rich range of phenomena, including electrochemical solid state amorphization, crystallization at full lithiation of a Li 15 Si 4 phase, hysteresis in the first lithiation−delithiation cycle, and highly anisotropic lithiation in crystalline samples. Very little is known about these processes at an atomistic level, however. To provide fundamental insights into these issues, we develop and apply a first principles, history-dependent, lithium insertion and removal algorithm to model the process of lithiation and subsequent delithiation of crystalline Si. The simulations give a realistic atomistic picture of lithiation demonstrating, for the first time, the amorphization process and hinting at the formation of the Li 15 Si 4 phase. Voltages obtained from the simulations show that lithiation of the (110) surface is thermodynamically more favorable than lithiation of the (100) or (111) surfaces, providing an explanation for the drastic lithiation anisotropy seen in experiments on Si micro-and nanostructures. Analysis of the delithiation and relithiation processes also provides insights into the underlying physics of the lithiation−delithiation hysteresis, thus providing firm conceptual foundations for future design of improved Si-based anodes for Li ion battery applications.
Two-Phase Electrochemical Lithiation in Amorphous Silicon
Nano Letters, 2013
Lithium-ion batteries have revolutionized portable electronics and will be a key to electrifying transport vehicles and delivering renewable electricity. Amorphous silicon (a-Si) is being intensively studied as a high-capacity anode material for nextgeneration lithium-ion batteries. Its lithiation has been widely thought to occur through a single-phase mechanism with gentle Li profiles, thus offering a significant potential for mitigating pulverization and capacity fade. Here, we discover a surprising twophase process of electrochemical lithiation in a-Si by using in situ transmission electron microscopy. The lithiation occurs by the movement of a sharp phase boundary between the a-Si reactant and an amorphous Li x Si (a-Li x Si, x ∼ 2.5) product. Such a striking amorphous−amorphous interface exists until the remaining a-Si is consumed. Then a second step of lithiation sets in without a visible interface, resulting in the final product of a-Li x Si (x ∼ 3.75). We show that the two-phase lithiation can be the fundamental mechanism underpinning the anomalous morphological change of microfabricated a-Si electrodes, i.e., from a disk shape to a dome shape. Our results represent a significant step toward the understanding of the electrochemically driven reaction and degradation in amorphous materials, which is critical to the development of microstructurally stable electrodes for high-performance lithium-ion batteries.
ACS applied materials & interfaces, 2014
Si thin films obtained by plasma enhanced chemical vapor deposition (PECVD) were used to investigate chemical and morphological modifications induced by lithiation potential and cycling. These modifications were thoughtfully analyzed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling, which allows to distinguish the surface and bulk processes related to the formation of the solid electrolyte interphase (SEI) layer, and Li-Si alloying, respectively. The main results are a volume expansion/shrinkage and a dynamic behavior of the SEI layer during the single lithiation/delithiation process and multicycling. Trapping of lithium and other ions corresponding to products of electrolyte decomposition are the major reasons of electrode modifications. It is shown that the SEI layer contributes to 60% of the total volume variation of Si electrodes (100 nm). The apparent diffusion coefficient of lithium (DLi) calculated from the Fick's second law directly from Li-io...
In situ atomic-scale imaging of electrochemical lithiation in silicon
Nature Nanotechnology, 2012
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.
Kinetics of Initial Lithiation of Crystalline Silicon Electrodes of Lithium-ion Batteries
2012
Electrochemical experiments were conducted on {100},{110}, and {111} silicon wafers to characterize the kinetics of the initial lithiation of crystalline Si electrodes. Under constant current conditions, we observed constant cell potentials for all orientations, indicating the existence of a phase boundary that separates crystalline silicon from the amorphous lithiated phase. For a given potential, the velocity of this boundary was found to be faster for {110} silicon than for the other two orientations.
The lithiation process and Li diffusion in amorphous SiO2 and Si from first-principles
Electrochimica Acta, 2020
Silicon is considered the next-generation, high-capacity anode for Li-ion energy storage applications, however, despite significant effort, there are still uncertainties regarding the bulk Si and surface SiO 2 structural and chemical evolution as it undergoes lithiation and amorphization. In this paper, we present first-principles calculations of the evolution of the amorphous Si anode, including its oxide surface layer, as a function of Li concentration. We benchmark our methodology by comparing the results for the Si bulk to existing experimental evidence of local structure evolution, ionic diffusivity as well as electrochemical activity. Recognizing the important role of the surface Si oxide (either native or artificially grown), we undertake the same calculations for amorphous SiO 2 , analyzing its potential impact on the activity of Si anode materials. Derived voltage curves for the amorphous phases compare well to experimental results, highlighting that SiO 2 lithiates at approximately 0.7 V higher than Si in the low Li concentration regime, which provides an important electrochemical fingerprint. The combined evidence suggests that i) the inherent diffusivity of amorphous Si is high (in the order 10 À9 cm 2 s À1-10 À7 cm 2 s À1), ii) SiO 2 is thermodynamically driven to lithiate, such that LieO local environments are increasingly favored as compared to SieO bonding, iii) the ionic diffusivity of Li in Li y SiO 2 is initially two orders of magnitude lower than that of Li y Si at low Li concentrations but increases rapidly with increasing Li content and iv) the final lithiation product of SiO 2 is Li 2 O and highly lithiated silicides. Hence, this work suggests that-excluding explicit interactions with the electrolyte-the SiO 2 surface layer presents a kinetic impediment for the lithiation of Si and a sink for Li inventory, resulting in non-reversible capacity loss through strong local LieO bond formation.
Advanced Energy Materials, 2019
to an unstable solid electrolyte interphase (SEI) [4-7] and large volumetric changes that lead to particle pulverization and loss of electrical contact between the active particles and the current collector. [8-12] Several approaches are being pursued to address these challenges, which include the following: electrolyte additives to improve SEI integrity, [5,7] electrode binders with enhanced mechanical [13] and chemical properties, [6] morphological engineering of Si particles (nanowires, nanotubes, porous particles, nanospheres, etc.), [12,14] limited-capacity cycling to limit volume expansion of the Si particles, [8,15] and developing electrodes that contain blends of Si and Gr materials. [3,7,11] The latter approach, coupled with SEI modifying additives such as fluoroethylene carbonate (FEC), has markedly improved electrode cycling performance compared to that of pure Si electrodes. [3,7,16] The potentials and kinetics for Li insertion and extraction from Si and Gr are known to be different. During electrochemical cycling of cells with a Li metal electrode ("half cells"), Si is active in the entire voltage range of 0-1.0 V, whereas Gr is active mainly below ≈0.25 V versus Li/Li + (abbreviated as V Li). [3,8,15] The relative lithiation/delithiation behavior of the Si and Gr components in blended electrodes is not well understood. Knowledge of Li distribution between the two active components is important as it determines the volume change of the electrode during electrochemical cycling (mainly due to Si particle expansion). One reason for the knowledge gap is the amorphization of crystalline Si that occurs during lithium insertion, which makes it difficult to track the evolution of the component by conventional techniques such as X-ray diffraction (XRD). In this study, operando energy dispersive XRD is used to quantify Li content of Gr in a 15 wt% Si-Gr composite electrode. In parallel, data obtained from operando studies on a Gr-only electrode are used to calibrate behavior of the Gr component in the blended electrode. By combining the knowledge of Gr lithiation with information on the coulometric capacity for the same cell obtained from electrochemistry, the Li content of the Si component can be inferred, and the fractional lithiation of each component is obtained. The knowledge gained from our study can be used to optimize the Si and Gr contents in the blended electrode, as well as to select cutoff capacities in limited-capacity schemes, which aim to limit electrode expansion by limiting expansion of the Si component. Additional Due to the high lithium capacity of silicon, the composite (blended) electrodes containing silicon (Si) and graphite (Gr) particles are attractive alternatives to the all-Gr electrodes used in conventional lithium-ion batteries. In this Communication, the lithiation and delithiation in the Si and Gr particles in a 15 wt% Si composite electrode is quantified for each component using energy dispersive X-ray diffraction. This quantification is important as the components cycle in different potential regimes, and interpretation of cycling behavior is complicated by the potential hysteresis displayed by Si. The lithiation begins with Li alloying with Si; lithiation of Gr occurs at later stages when the potential dips below 0.2 V (all potentials are given vs Li/Li +). In the 0.2-0.01 V range, the relative lithiation of Si and Gr is ≈58% and 42%, respectively. During delithiation, Li + ion extraction occurs preferentially from Gr in the 0.01-0.23 V range and from Si in the 0.23-1.0 V range; that is, the delithiation current is carried sequentially, first by Gr and then by Si. These trends can be used for rational selection of electrochemical cycling windows that limits volumetric expansion in Si particles, thereby extending cell life.
Deformation and failure mechanisms of electrochemically lithiated silicon thin films
A fundamental understanding of mechanical behavior of a Li-Si system is necessary to address the poor mechanical integrity of amorphous silicon (a-Si) electrodes, in order to utilize their enormous capacity in Li-ion batteries. In this work, deformation and failure mechanisms of electrochemically lithiated a-Si thin films were investigated using nanoindentation and molecular dynamics simulation techniques. The cracking observed in the a-Si thin films after the initial lithiation-delithiation cycle is associated with the tension stress developed when constrained by the substrates. The MD simulations provide an atomistic insight on the origin of plasticity and transition of fracture mechanisms with increasing lithium concentration in the electrode. Both experiment and the MD simulations indicate reduced strength, elastic modulus but increased ductility in the a-Si films after the full lithiation-delithiation cycle, as a result of increased disorder in the microstructures. Also, the mapping of void nucleation and growth indicates different failure modes in pristine and delithiated a-Si.