Evaluating the Electrolyte Consumption and Cycling Performance of Practical Silicon-Graphite Electrodes
Authors: Oswald Stefan , Wetjen Morten , Pritzl Daniel , Gasteiger Hubert A. 
Published: February 2020

Silicon-based anodes are an integral part of lithium-ion cell chemistries that reach the specific energy target of >350 Wh kg-1 on a cell-level.1 Nonetheless, commercialization of these electrodes is still hampered, because of two major drawbacks: (i) The mechanical degradation of the electrodes due to volumetric changes of the silicon particles upon repeated (de-)lithiation,2 and (ii) the ongoing electrolyte consumption caused by side reactions at the silicon-electrolyte interface.3

During the previous two decades, researchers pursued various strategies to overcome these drawbacks, e.g., by use of active/inactive alloy-based electrodes,4 silicon thin-films,5 and carbon-coated silicon-nanopowders.6 However, there remains a need for investigations that (i) use scale-able electrode concepts and (ii) allow to derive implications for lithium-ion full-cells with limited amount of active lithium and electrolyte.

In the present study, we investigate composites and mixtures of commercially relevant silicon-graphite electrodes regarding their electrolyte consumption and cycling performance, using industrially scale-able materials and processes. Hence, we prepared electrodes with a practical areal capacity of ~2.0 mAh cm-2, comprising 90 wt% active materials (8-25 wt% silicon, all materials from commercial suppliers), 7 wt% lithium poly(acrylic acid) binder, and 3 wt% C65 conductive carbon.

The surface area and the composition of the pristine silicon-graphite materials was investigated by means of BET measurements and thermal-gravimetric analysis (TGA). After the coating procedure, the electrodes were first investigated in terms of their morphology and then regarding their electrochemical behavior, using differential capacity analysis of galvanostatic measurements in coin-cells against lithium metal counter electrodes.

To evaluate the electrolyte consumption and cycling performance of the different active materials, the silicon-graphite electrodes were cycled in pseudo full-cells against capacitively oversized LiFePO4 electrodes (3.5 mAh cm-2, Custom Cells, Germany).7 As electrolyte solution, 1 M LiPF6 dissolved in a 3:7 (v:v) mixture of ethylene carbonate:ethyl methyl carbonate (LP57) with 5 wt% fluoroethylene carbonate (FEC) as additive was used. After 100 cycles, the silicon-graphite electrodes were harvested from the cells and examined in terms of their thickness and morphological changes by scanning electron microscopy (SEM). Finally, the loss of FEC upon cycling was quantified by post-mortem 19F-NMR analysis of the residual electrolyte to estimate the electrolyte consumption upon cycling.

This contribution concludes with a critical review of the properties (e.g., BET surface area, particle size) and the performance of the different silicon-graphite active materials. By benchmarking the results with conventional graphite electrodes, our research helps to relate the electrochemical results to the properties of the silicon-graphite active materials and thus support future material selections. Further, we use these data to estimate the lifetime of commercial lithium-ion batteries, comprising these silicon-graphite materials, as a function of the amount of electrolyte and the positive electrode capacity.

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