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

The kinetics and mechanisms of phase transitions that occur during lithiation of amorphous silicon thin films were extensively studied using electrochemical techniques and transmission electron microscopy. Measurements of current made while changing applied potentials at a constant rate (cyclic voltammograms) showed peaks that correspond to the formation of two amorphous Li-Si alloys with different stoichiometries. Peaks associated with these phase transitions were also observed in current-time measurements made in potentiostatic experiments. The times at which currents reached maxima were found to be independent of film thickness, suggesting that the phase transitions occur throughout the volume of the films. Analysis of current-time measurements using the Johnson-Mehl-Avrami-Kolmogorov method indicate that the transitions occur through a first-order nucleation and growth process. This conclusion is supported by cross-section transmission electron microscopy in which high-energy electron-beam-induced sputtering of lithium led to contrast between the parent and product phases. In contrast with other studies, the polyamorphic amorphous-to-amorphous first-order phase transitions observed in this study are driven by changes in composition rather than changes in pressure or temperature. The potentiostatic techniques employed in this study can be used to characterize the nature and kinetics of phase transitions that occur in other electrode materials for lithium-ion batteries.

Figure 1

CV (cyclic voltammogram) plots for lithiation and delithiation of a-Si in cycles 2–4. The scan rate was 35 μV/s and the sample was cycled between 0.02 and 1 V.

Figure 2

(a) Schematic of a two-step potentiostatic test, and the current-time plots at different $V_2$ for (b) $V_1=270\mathrm{mV}$ and (c) $V_1=150\mathrm{mV}$.

Figure 3

Current-time plots for (a) $V_1=270\mathrm{mV}, V_2=200\mathrm{mV}$ and (b) $V_1=150\mathrm{mV}, V_2=80\mathrm{mV}$ in a-Si films with different thicknesses.

Figure 4

(a) Voltage-time plots for a potentiostatic intermittent titration technique (PITT) experiment, and (b) the corresponding current-time plots for an a-Si film.

Figure 5

Schematics of three phase-transition models illustrated for cross sections of Si films and the corresponding current-time curves for two samples of different thickness $\left(H_2>H_1\right)$: (a) phase propagation with a sharp interface and complete transition at the end [25], (b) phase transition involving nucleation and growth that initially occurs near the top surface of the film and then a nucleation front propagates through the film, and (c) nucleation and growth that occurs uniformly through the film. In cases (b) and (c), two phases coexist at the end of the transition. The newly formed Li-rich phase is indicated by the darker blue regions.

Figure 6

(a) Current-time plot at $V_2=220\mathrm{mV}$ after equilibration at $V_1=270\mathrm{mV}$. Red dots correspond to TEM samples I, II, and III that were lithiated potentiostatically for different lengths of time. (b)–(d) Original cross-section TEM images of samples I, II, and III, and (e)–(g) the corresponding labeled images. (h) Transition fraction–time plots calculated from potentiostatic tests and TEM images. See Supplemental Material [54] for image processing details.

Figure 7

(a) Current-time plot for $V_2=220\mathrm{mV}$ after equilibration at $V_1=270\mathrm{mV}$. Different colors indicate different stages of the phase transition. (b) Experimental and fitted current-time plots. The black plot is the temporal evolution of the total current minus the background current, and the orange plot is the fitted current-time curve using the JMAK model. The red dot represents the inflection point.

Figure 8

The Avrami constant $n$ as a function of the voltages of potentiostatic holds ${\mathrm{V}}_2$ after equilibration at (a) $V_1=270\mathrm{mV}$ and (b) $V_1=150\mathrm{mV}$, and the Avrami kinetic parameter $b$ as a function of $V_2$ for (c) $V_1=270\mathrm{mV}$ and (d) $V_1=150\mathrm{mV}$ for films with different thicknesses. The values of $n$ and $b$ were determined by fitting the current-time curves using the JMAK model.

Figure 9

(a) TEM image of a sample potentiostatically lithiated at $V_2=220\mathrm{mV}$ after equilibration at $V_1=270\mathrm{mV}$ and held at $V_2$ for a long enough time to complete the phase transition. (b) Schematic of a modified volume transition model incorporating step-by-step nucleation and growth.