Operando Electrochemical Dilatometry of Si-Based Electrodes for Lithium-Ion Cells

Abstract

Silicon is a promising next-generation anode material for Li-ion batteries, given its high specific capacity, abundance and low working potential. However, its application in commercial cells has been hindered by the large volumetric changes that occur during the Li (de)alloying process. While theoretical calculations predict that the full lithiation of a silicon particle would lead to ~300% expansion,1 experiments suggest that the material can undergo even larger dimensional variations due to morphological changes during repeated cycling.2 Most of the degradation mechanisms associated with Si-based electrodes, which are responsible for the poor cycling stability and rapid capacity decay, derive from the cyclic swelling/contraction effect. Understanding how the particle-level expansion projects into the electrode scale is hence a very relevant topic, specifically considering that composite electrodes are a complex combination of pores and both active and inactive materials.In this study, we use operando electrochemical dilatometry to quantify the thickness variation of Si-based electrodes during cycling. An example plot from a 70 wt% Si electrode is shown in Figure 1. The electrode expands to 300% of its initial thickness upon full lithiation. However, the swelling shows a nonlinear behavior being much slower during the initial 1000 mAh/g. At this point, the thickness change is only 17% although ~40% of the electrode capacity is utilized. During this regime, porosity and particle rearrangement within the anode may be able to partially accommodate the particles expansion. Past this stage, however, there is a steep increase in electrode dilation, which often results in coating delamination, as confirmed by post-mortem inspections.Such quantitative analyses provide important insights into the relationship between electrode capacity and expansion. More importantly, these results can be applied to inform cell design, provide guidelines for selection of cell cutoff voltages, and estimate the effect of anode utilization on cell performance. Obtaining reliable dilatometry information is challenging: hence, strategies for designing experiments that yield reproducible results will also be discussed during the presentation. REFERENCES 1. Kim et al., Journal of Physical Chemistry C, 115, 2514 (2011). 2. Wetjen et al., Journal of The Electrochemical Society, 165, A1503 (2018).Acknowledgement: We are grateful for support from B. Cunningham at the U.S. DOE Office of Vehicle Technology. This document has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. Figure 1