In Situ measurements of Stress and Potential Evolution during Self Discharge of a Lithiated Silicon Electrode

Abstract

In this work, we report direct/in situ measurements of stress and potential evolution during self-discharge of a fully lithiated silicon electrode. Parasitic reactions, typically attributed to the formation of the solid-electrolyte-interphase (SEI) layer on the surface of the silicon electrode, cause the self-discharge leading to the loss of cyclable lithium ions from the electrode and irreversible capacity loss. These parasitic reactions continuously occur when the electrode potential is below the equilibrium potential (typically 0.8V vs. Li/Li+) for SEI formation, and when the surface is electronically conductive. We previously reported on coupled electrochemical-mechanical measurements in the Li-Si binary system between pure Si and Li15Si4 with the following key observations: the system undergoes cyclic compressive (up to -2GPa) and tensile stresses (up to +2GPa) during electrochemical lithiation and delithiation, respectively, with extensive plastic flow and associated mechanical dissipation1; the biaxial stress and the electrode potential are strongly coupled2; the electrode softens upon lithiation and toughens upon delithiation3; and the energy losses due to mechanical dissipation are comparable to the sum of kinetic/polarization, and ohmic losses4. Using the substrate-curvature measurement technique, we measured the stress and potential evolution during self-discharge (due to parasitic reactions) of a fully lithiated thin-film silicon electrode, and compare it to same measurements made during galvanostatic delithiation. Upon self-discharge, the stress of a fully lithiated electrode evolves from -1.2 GPa (compressive) towards a state of zero stress, and continues to become tensile (+0.5 GPa). The evolution from -1.2 GPa towards zero stress and continual increase in the tensile direction is caused by the removal of lithium ions from the electrode driven by the parasitic reactions5, and tensile stresses typically cause cracking and damage in electrodes. Figure 1 shows (a) the current density, (b) potential vs. Li/Li+, and (c) the electrode stress during a galvanostatic lithiation/delithiation cycle (in red), and during galvanostatic lithiation followed by self-discharge and galvanostatic delithiation (in blue).We also quantified the rates at which the parasitic reactions occur by measuring both potential and stress evolution during self-discharge at various states of charge, SOCs (i.e., by varying the concentration of lithium in the silicon electrode via galvanostatic lithiation, followed by open-circuit measurements). We show that the resulting parameters are useful in predicting changes in electrode stress and its evolution during the self discharge at various SOCs. We will discuss why these measurements are useful in the context of storing fully charged lithium-ion batteries on the shelf for long periods of time. Because of self-discharge-induced mechanical damage, for batteries made with a large-volume-expansion electrodes, it is better to store them at or near a state of zero stress than at a higher SOC.Acknowledgements:This work was supported at the Indian Institute of Science - Bangalore, by XII Plan grant (#12-0509-0457-01), and at Faraday Laboratory LLC by financial support from Unify Inc. (04UNIFY04302018 onward). PG gratefully acknowledges financial support through the Kishore Vaigyanik Protsahan Yojana Scholarship (KVPY, 2012-2017).