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

Jinghui Miao,Carl V Thompson

doi:10.1149/2.1011803jes

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 LixSi 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 LixSi phase rather than reaction at the lithiation front. The Li diffusivity in the lithiated phase was determined to be in the 10−13 cm2/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.

Highlights

The MIT Faculty has made this article openly available
The disappearance of peak 1 in following cycles is consistent with previous results that indicate that a significant structural change occurs irreversibly in amorphous Si in the initial cycle.[25,27]
During potentiostatic testing of films, a kink feature is observed during the first lithiation process

Summary

Introduction

The MIT Faculty has made this article openly available. Please share how this access benefits you. 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. Equilibrium titration experiments at 415◦C show that four crystalline lithium silicide (LixSi) phases form (Li12Si7, Li7Si3, Li13Si4 and Li22Si5).[7,8,9] At room temperature, 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 Li15Si4 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 associated with higher barriers to nucleation of the crystalline phases.[13] Amorphous alloy phases with different stoichiometries have been associated with features in discharge and CV curves for Si,[14,15] and their local atomic structures have been studied using nuclear magnetic resonance (NMR), XRD and Mossbauer spectroscopy.[16,17,18,19] Distinct local atomic configurations of Li and Si have been identified at different stages of lithiation. Due to the large structural change from the pristine crystalline Si phase to an amorphous lithiated Si phase, the first lithiation cycle for c-Si has been widely accepted to follow a first-order phase transition mechanism, characterized by motion of a sharp interface between the crystalline and amorphous phases

Results

Discussion

Conclusion