Electrogravimetry and Structural Properties of Thin Silicon Layers Deposited in Sulfolane and Ionic Liquid Electrolytes

Abstract

Potentiostatic deposition of silicon is performed in sulfolane (SL) and ionic liquid (IL) electrolytes. Electrochemical quartz crystal microbalance with damping monitoring (EQCM-D) is used as main analytical tool for the characterization of the reduction process. The apparent molar mass (M app ) is applied for in situ estimation of the layer contamination. By means of this approach, appropriate electrolyte composition and substrate type are selected to optimize the structural properties of the layers. The application of SL electrolyte results in silicon deposition with higher efficiency compared to the IL 1-butyl-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMP][TFSI]. This has been associated with the instability of the IL in the presence of silicon tetrachloride and the enhanced incorporation of IL decomposition products into the growing silicon deposit. X-ray photoelectron spectroscopy (XPS) analysis supports the results about the layer composition, as suggested from the microgravimetric experiments. Attention has been given to the impact of practically relevant substrates (i.e., Cu, Ni, and vitreous carbon) on the reduction process. An effective deposition can be carried out on the metal electrodes in both electrolytes due to accelerated reaction kinetics for these types of substrates. However, on vitreous carbon (VC), a successful reduction of SiCl 4 can only be accomplished in the IL, while the electroreduction process in SL is dominated by the decomposition of the electrolyte. For short deposition times, the scanning electron microscopy (SEM) images display rough morphologies in the nanometer range, which evolve further to structures with increased length scale of the surface roughness. The development of a rough interface during deposition, resulting in QCM damping at advanced stages of the process, is interpreted by a model accounting for the resistive force caused by the interaction of the liquid with a nonuniform layer interface. By using this approach, the individual contributions of the surface roughness and viscoelastic effects to the measured damping values are estimated.

Reversible Sodiation of Electrochemically Deposited Binder‐ and Conducting Additive‐Free Si–O–C Composite Layers

“…[ 21,34–36 ] The experimentally obtained damping observed at the end of the deposition period conforms very well with the theoretically predicted values corresponding to these roughness parameters. [ 21 ]…”

“…For the SL electrolyte, used for the Si deposition at 298 K, one obtains δ = 500 nm. [ 21 ] Therefore, the electrolyte, trapped in surface cavities with dimension under this limit, will cause a Sauerbrey‐like frequency shift, characteristic for a rigid layer. [ 31 ] However, this quantification will not correspond to the mass of the deposited Si–O–C layer.…”

“…The Si–O–C material electrodeposited from SL displays an enhanced electrochemical–mechanical stability upon lithiation, which can be ascribed to effects of the chemical composition. [ 20,21 ]…”

“…[17] Along with other effective methods, silicon-containing materials have been deposited also electrochemically, at room temperature, in different organic-based electrolytes. [18][19][20][21][22][23] The analysis of their chemical composition showed that apart from elemental silicon, depending on the electrolyte type, the deposits typically contain considerable amounts of additional components, originating from the decomposition of the corresponding solvent and supporting electrolyte salt. [20][21][22][23] According to the authors' knowledge, the sodiation behavior of this type of silicon-containing composite materials has not been explored so far.…”

“…In our previous works, we found that due to the integrated extra components, the viscoelasticity of the electrodeposited Si-based layer is substantially increased. [20][21][22] This effect plays a positive role for achieving an improved cycling stability of the deposits in electrolytes for LIBs. [19,20] Related to the advanced application of the Si-O-C material, our particular attention has been focused on the sulfolane (SL)-based electrolytes as electrochemically stable, low-cost, and environmentally friendly alternative to the conventional organic solvent-based ones.…”