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.
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(6) Wetjen, M.; Jung, R.; Pritzl, D.; Gasteiger, H. A. ECS Meet. 230 2016, Abstr. #280.
The German Federal Ministry for Economic Affairs and Energy is acknowledged for funding (“LiMo” project with funding number 03ET6045D).