Electrochemical Nucleation of Silicon in Ionic Liquid-Based Electrolytes

Abstract

Silicon is widely applied in a high number of industrial technologies, including different types of electronics, solar energy conversion and electrochemical energy storage. This extensively studied material has an exceptionally high theoretical capacity, more than ten times higher compared to the currently used graphite anodes in Li-ion batteries [1]. However, its main disadvantage is the huge volume expansion upon alloying with lithium, inducing disintegration of the electrode material, loss of electrical contact, poor reversibility and a rapid capacity fade [1]. It has been established that Si applied in a nanostructured or nanoparticulate form displays an enhanced electrochemical stability. Therefore, advanced production technologies have to be implemented in order to obtain cost-effectively the required electrochemically stable Si morphologies.Electrochemical methods offer a simple and low-cost deposition process. Furthermore, the electrodeposition is a versatile technique, allowing control of the morphology by adjusting the experimental conditions and obtaining deposits on different substrate geometries. However, due to the very low reduction potential and high reactivity of silicon precursors, it is impossible to deposit this element in aqueous electrolytes. Therefore, a broad variety of alternative media, including high temperature molten salts (HTMS) [2], conventional organic solvents [3] and ionic liquids (ILs) [4] has been researched.It is established that the initial stages of electrodeposition play an important role for the physical properties, stability and functional behavior of the final deposit. For example, homogeneous distribution of a high number density of small Si crystals would be beneficial for the electrochemical stability of the deposit during lithiation. Therefore, the nucleation and growth process of silicon deserves a special attention. Initial studies on the formation of Si nuclei in HTMS have been recently reported [2]. It was found that the diffusion limited nucleation process in HTMS (i.e. KF-LiF-K2SiF6) is instantaneous, followed by 3D growth [2]. However, due to the high temperatures (T > 800oC) and difficulties to perform the Si deposition economically in these electrolytes the research interest has turned to low temperature molten salts (LTMS).In our study we analyze the initial stages of Si electrodeposition in IL electrolytes by interpreting the chronoamperometric data (fig.1a) with a model accounting nucleation and growth under diffusion limitation, established by Scharifker et al. [5-7], eq.1. Vitreous carbon was selected as an inert substrate for the experiments and 1-butyl-1-methylpyrrolidinium bis (trifluoromethylsulfonyl) imide [BMP][TFSI] containing SiCl4 was chosen as an electrolyte.The current transients were quantitatively interpreted by a non-linear fitting procedure, applying a model equation for three-dimensional nucleation, eq.1. Eq.1 was modified with a term describing the surface charging phenomena, observed at the beginning of the silicon reduction, approximated to an exponential decay after Langmuir adsorption kinetics, eq.2. To prove the validity of this procedure, we extracted the charging currents (fig.1b) and fitted the resulting curves with the non-modified equation for 3D nucleation, where the respective output parameters remained nearly identical. According to eq.(3), the data for P1 should be constant for all fitted curves (i.e. potentials) since this parameter does not depend on other variables. This allows the value of the diffusion coefficient D =1.3×10−8 cm2 s−1 to be calculated. D was additionally confirmed by the analysis of the current maxima [5,6]. The data for the nucleation frequency, A and the number density of active sites, N0 obtained from the best fit are presented in Fig. 1c and show the typically observed linear increase as a function of the overpotential,η [7].Furthermore, we characterized the nucleation process by analyzing the current maxima in dimensionless coordinates (fig. 1d). According to the analysis the initial stages of the Si phase formation display an instantaneous behavior, showing a progressive character with decreasing the applied overpotential. The theoretically evaluated values for N0 correlate well with the crystal number density N, acquired by scanning electron microscopy. The talk will cover the main aspects of the electrochemical nucleation of Si in non-aqueous electrolytes and will provide a discussion on the major challenges towards understanding this process.