Investigation of the Porosity and Tortuosity of Silicon-Graphite Electrodes in Lithium-Ion Batteries

Abstract

Silicon-based electrodes are promising candidates for the next generation of Lithium-ion batteries with cell-level volumetric energy densities above 700 Wh L-1 cell.1,2 However, the practical energy density of these electrodes is strongly influenced by the electrode porosity and the volume expansion during repeated charge-discharge.3 In addition, the electrode’s rate capability is largely determined by its tortuosity, which continuously increases upon cycling as a consequence of ongoing electrolyte decomposition at the silicon-electrolyte interface.4 In the present study, we investigate the influence of the porosity and tortuosity on the cycling performance of silicon-graphite electrodes. Hence, we prepared electrodes with a practical areal capacity of 4.0 mAh cm-2, comprising 35 wt% nanometer-sized silicon and 58 wt% micron-sized graphite particles. Lithium poly(acrylic acid) binder and C65 conductive carbon accounted for the remaining 7 wt%. By use of a hydraulic press, the electrode porosity was adjusted to values ranging from 30% to 50%. As electrolyte solution, 1 M LiPF6 dissolved in a 3:7 (w:w) mixture of ethylene carbonate:ethyl methyl carbonate (LP57) with 5 wt% fluoroethylene carbonate (FEC) as additive was used. First, the tortuosity and MacMullin number of the pristine silicon-graphite electrodes were determined by use of impedance spectroscopy on symmetrical cells in blocking conditions.5 The electrode polarization and rate capability of these electrodes were then investigated in coin-cells with lithium metal foil as counter electrode. After 50 cycles, the silicon-graphite electrodes were harvested from the cells and examined in terms of their thickness and morphological changes by cross-sectional scanning electron microscopy. Finally, the FEC consumption upon cycling was quantified by post-mortem 19F-NMR analysis of the residual electrolyte to estimate the pore volume of the electrodes that was occupied by electrolyte decomposition products.6 This contribution concludes with a comparison of silicon-graphite electrodes with different porosities and the quantification of the resulting degradation phenomena. Our research helps to better understand the limiting processes in silicon-graphite electrodes upon cycling and provides useful parameters for the modelling and simulation of these electrodes.