Studying Degradation of Micro- and Nano-Scale Silicon in Si/ Rgo-Anode Materials for Lithium-Ion Batteries – Towards a Fair Comparison

Abstract

An attractive approach to increase the Li+-storage capacity of anode materials in Lithium-ion batteries (LiBs), is to replace or combine the routinely-applied graphite (372 mAh g-1) with higher-capacity materials. Among others, silicon (Si) with a theoretical capacity of 3579 mAh g-1 is a promising candidate [1]. Silicon is not yet been used as an anode material in its pure form, due to its low conductivity and volume increase of up to 300 % during lithiation. This two-step expansion (1st: Si + 2 Li+ + 2 e- → SiLi2.0; 2nd: SiLi2.0 + 1.5 Li+ + 1.5 e- → SiLi3.5) is a source of mechanical stress that can ultimately lead to battery failure. Furthermore, the initial lithiation of crystalline silicon (c-Si + 3.5 Li+ + 3.5 e- → SiLi3.5) causes an amorphization of the pristine crystalline lattice. This structural change causes further degradation of the material, which can hardly be avoided [2].The main degradation mechanisms resulting from expansion and amorphization are I) The uncontrolled formation of a passivation layer due to the cyclic volume change taking place during (de-)lithiation, which leads to a continuous break-up and reformation of the solid electrolyte interphase (SEI). II) The pulverization of particles resulting in mechanical stress and the electrical isolation of particles or particle fragments delaminating from the electrode [3]. The first mechanism discussed is more dominant for nanometer scaled particles (nm-Si), while the second mechanism occurs mostly for particles in the micrometer range (µm-Si).Our approach combines, nm- or µm-sized Si particles with different matrix materials. By incorporating the particles into an either graphitic or soft carbon-like support, we aim to increase the conductivity of the electrode material and gain additional buffer space thereby reducing interparticle mechanical stress [4]. On the one hand, the systems of carbon-based anode materials are already well understood in terms of electrode composition, processing parameters, electrolyte compatibility, formation of a stable SEI and the resulting high cycling stability. On the other hand, the observed degradation mechanisms will occur with different predominance in different Si/C composites depending on the Si particle size and the chosen carbon matrix. However, the latter is not completely understood. We will address these issues by comparing different Si/C-composites consisting of nm-Si-particles or µm-Si particles in a reduced graphite oxide (rGO) matrix and in a graphite-matrix.Electrochemical methods, such as galvanostatic cycling, impedance spectroscopy and dq/dU-analysis in 2- and 3-electrode setups, were coupled with optical and spectroscopical techniques (e.g. XRD, FTIR, SEM) to trace back the electrochemically measurable degradation of the electrode to structural changes during formation and cycling. Particular attention was paid to the localization and analysis of defect areas with increased degradation by correlating the optical and spectroscopical results data with the lithiation and delithiation processes. Considering the different SoC levels, we can identify the ongoing aging of the different electrode materials. Based on this, we propose the use of soft carbon-like structures such as rGO as a matrix material for Si/C composites due to its higher cycling stability under the investigated conditions.