Kinetic Study of the Initial Lithiation of Amorphous Silicon Thin Film Anodes

Abstract

The mechanisms and kinetics of lithiation and delithiation of amorphous silicon were investigated using potentiostatic techniques and thin films of different thickness, with a focus on the initial lithiation process that occurs in the first cycle. In potentiostatic tests, distinct kinks were observed in the current vs. time curves, and the time at which the kink occurred increased for thicker films. This behavior can be explained using a model in which a sharp interface between an amorphous Li x Si phase and Li-saturated amorphous Si propagates through the film. Using this model, the rate-limiting process was determined to be diffusion of Li in the Li x Si phase rather than reaction at the lithiation front. The Li diffusivity in the lithiated phase was determined to be in the 10 −13 cm 2 /s range, independent of film thickness above 135 nm. The thin-film potentiostatic technique used in this study should prove useful in investigation of the mechanisms and rate parameters for other phase transitions that occur during lithiation of silicon and for kinetic studies of other electrode materials. Li-ion rechargeable batteries have been the focus of much research and development over recent decades. They power products ranging from electric vehicles to miniaturized electronic and medical devices. Due to its largest known capacity (8375 Ah/cm 3 , 3579 Ah/kg), Si is viewed as a promising anode material for batteries with high energy and power densities. Its abundance and relatively low cost also make it a practically feasible anode material.1 However, Si anodes suffer from capacity fade due to the very large volume expansion (up to 400%) 2 that occurs during lithiation and the consequent mechanical degradation it causes.3 In storing lithium, silicon goes through a sequence of structural transitions that affect the rate at which mechanical stress develops. 4,5 This, in turn, affects the degree to which mechanical stresses can be accommodated through plastic deformation rather than fracture. 6 Hence, a detailed understanding of the nature and kinetics of structural transitions that occur during lithiation and delithiation of silicon should provide insight into means of mediating the effects of mechanical stress on the cyclability of Si anodes.Electrochemically driven reactions between Li and Si have been studied under a variety of conditions, including at elevated and room temperature. Equilibrium titration experiments at 415• C show that four crystalline lithium silicide (Li x Si) phases form (Li 12 Si 7 , Li 7 Si 3 , Li 13 Si 4 and Li 22 Si 5 ).7-9 At room temperature, however, it is found using X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) that the equilibrium crystalline Li-Si phases are absent and the crystalline Li 15 Si 4 phase is observed only at the highest levels of lithiation.10 At lower levels of lithiation, metastable amorphous phases are formed instead.11 This phenomenon is similar to chemically driven solid-state amorphization reactions, 12 which are thought to be assoc...

First-order amorphous-to-amorphous phase transitions during lithiation of silicon thin films

“…The equilibrium crystalline phases are bypassed due to higher barriers to their nucleation compared with those of the metastable amorphous phases [16]. Similar phenomena are also observed in other alloying anodes, as well as LiMPO 4 [17,18] and Li 2 MSiO 4 [19] cathodes, for varying metals M. The mechanism of the first-cycle lithiation of Si has been intensively studied using in situ transmission electron microscopy (TEM) [20][21][22], X-ray reflectivity (XRR) [23,24] and potentiostatic experiments (chronoamperometry) [25]. A consensus has been reached that a first-order phase transition occurs in the initial lithiation of Si, featuring the irreversible propagation of a sharp interface between the initial and lithiated amorphous phase.…”

“…3. In a previous study of the first-cycle lithiation of Si [25], we found that the time to complete the transition strongly depended on film thickness, indicating that the irreversible phase transition initiated at the Si/electrolyte surface and propagated through the thickness of the film. In the present case, however, while the peak and total currents increase with film thickness as expected, t peak is approximately the same for all film thicknesses.…”

“…5. Figure 5(a) is the lithiation mechanism observed for the first-cycle lithiation in a-Si and c-Si [20,21,23,25], and functions as a reference. The new phase (in dark blue) forms as a continuous layer at the top surface of the film early in the lithiation process.…”

Improvement of the Cycling Performance of Aluminum Anodes through Operando Light Microscopy and Kinetic Analysis

“…The model of Deal and Grove, which was developed to describe the growth of the oxide layer on silicon surfaces, has been well adapted to model phase transformations during electrochemical lithiation. Miao and Thompson developed a kinetic model based on the Deal–Grove model (which applies Fick's law for the case of a steady‐state flux) to elucidate the initial lithiation of amorphous silicon thin film anodes for LIBs . Another study also presented a multi‐stage model that illustrates the propagation of the Li x Si phase, the reaction rates at the interface, and the lithium diffusion behavior in crystalline silicon .…”

“…Miao and Thompson developedak inetic model based on the Deal-Grove model (which applies Fick's law for the case of as teady-state flux) to elucidate the initial lithiation of amorphous silicon thin film anodes for LIBs. [12] Another study also presented am ulti-stage model that illustrates the propagation of the Li x Si phase, the reaction rates at the interface, andt he lithium diffusion behavior in crystalline silicon. [13] For Al anodes, however,l ess information can be found regarding the kinetics of the electrochemical formation of Li-Al alloy at room temperature.…”