Exploring the behaviour of Silicon by empirical analysis of galvanostatic charge-discharge curves

Abstract

The capability of battery materials to deliver not only high lithium (Li) storage capacity, but also the ability to operate at high charge/discharge rates is an essential property for development of new batteries. First, the influence on the charge/discharge rate behaviour of substoichiometric concentrations of phosphorus (P) in silicon (Si) nanoparticles was studied. The results revealed an increase in rate capability as a function of the P concentration between 0 and 5.2 at%, particularly during delithiation. The stoichiometry of the nanoparticles was found to strongly affect the formation of the Li3.5Si phase during lithiation. Cyclic stability experiments demonstrated an initial increase in capacity for the SiPx materials. Galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) demonstrated the increased Li diffusivity with inclusion of P. Density functional theory and ab initio molecular dynamics were deployed to provide a rationale for the electrochemical behaviour of SiPx. This study revealed the need for further studies into the behaviour of pure Si, given that the galvanostatic charge-discharge (GCD) behaviour of Si is known to depend strongly on morphology, cycling conditions and electrochemical environment. One common method for analysing GCD curves is through differential capacity, but the data processing required necessarily degrades the results. We therefore present a method of extracting empirical information from the delithiation step in GCD data for Si at C-rates above equilibrium conditions. We find that the function is able to quickly and accurately determine the best fit to historical half-cell data on amorphous Si nanowires and thin films, and analysis of the results reveals that the function is capable of distinguishing the capacity contributions from the Li3.5Si and Li2Si phases to the total capacity. The method is can also pick up small differences in the phase behaviour of the different samples, making it a powerful technique for further analysis of Si data from literature. The method was also used for predicting the size of the reservoir effect (the apparent amount of Li remaining in the electrode), making it a useful technique for quickly determining voltage slippage and related phenomena. This work is presented as a starting point for more in-depth empirical analysis of Si GCD data. However, half cells commonly used for Si studies are limited by polarization of the Li counter electrode, especially at high Si loading. To study the interplay between Si and Li electrodes, a set of EIS spectra are generated using cycled Si half cells at four different potentials in the charge-discharge profile, and then repeated using symmetric Si/Si and Li/Li cells assembled from half cells cycled to equivalent stages in the cycle. Distribution of relaxation times (DRT) analysis is used to design equivalent circuits (ECs) for both Si/Si and Li/Li symmetric cells incorporating both electrolyte and electrode-related diffusion, and these are applied to the half cells. The results demonstrate that the behaviour of half cells is dominated by the solid electrolyte interphase (SEI) impedances at the Li counter electrode at the low and high potentials where the Li+ mobility signal in Si is limited, while the Si electrode is dominant at intermediate potentials where the signal from mobile Li+ is strong. EIS studies of Si half cells should therefore be performed at intermediate potentials, or as symmetric cells. Finally, we applied the empirical fitting function presented earlier to aid in the analysis of GCD data of commercial Si half-cells with high loading. We find that the fitting procedure is capable of detecting dynamic changes in the cell, such as reversible capacity fade of the Si electrode. This fading is found to be due to the highly lithiated Li2Si ←↽−−⇀→ Li3.5Si phase and that the behaviour is strongly dependent on the potential of this phase. EIS reveals that the Si electrode is responsible for the reversible behaviour due to progressive loss of Li+ leading to increasing resistance. To complete the characterization SEM/EDX and XPS are also employed to determine the origin of the irreversible resistance growth on the Si electrodes.