Vijay Anand Sethuraman

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). References: V.A. Sethuraman, M.J. Chon, M. Shimshak, V. Srinivasan, P.R. Guduru, J. Power Sources, 195, 5062 (2010). V.A. Sethuraman, V. Srinivasan, A.F. Bower, P.R. Guduru, J. Electrochem. Soc., 157, A1253 (2010). V.A. Sethuraman, M.J. Chon, N. Van Winkle, PR. Guduru, Electrochem. Comm., 12, 1614 (2010). V.A. Sethuraman, V. Srinivasan, J. Newman, J. Electrochem. Soc., 160, A394 (2013). S.P.V. Nadimpalli, V.A. Sethuraman, S. Dhalavi, B. Lucht, M.J. Chon, V.B. Shenoy, P.R. Guduru, J. Power Sources, 215, 145 (2012). Figure 1

Understanding the role of elastic strain in modifying catalytic reaction rates is crucial for catalyst design, but experimentally, this effect is often coupled with a ligand effect. To isolate the strain effect, we have investigated the influence of externally applied elastic strain on the catalytic activity of metal films in the hydrogen evolution reaction (HER). We show that elastic strain tunes the catalytic activity in a controlled and predictable way. Both theory and experiment show strain controls reactivity in a controlled manner consistent with the qualitative predictions of the HER volcano plot and the d‐band theory: Ni and Pt's activities were accelerated by compression, while Cu's activity was accelerated by tension. By isolating the elastic strain effect from the ligand effect, this study provides a greater insight into the role of elastic strain in controlling electrocatalytic activity.

Understanding the role of elastic strain in modifying catalytic reaction rates is crucial for catalyst design, but experimentally, this effect is often coupled with a ligand effect. To isolate the strain effect, we have investigated the influence of externally applied elastic strain on the catalytic activity of metal films in the hydrogen evolution reaction (HER). We show that elastic strain tunes the catalytic activity in a controlled and predictable way. Both theory and experiment show strain controls reactivity in a controlled manner consistent with the qualitative predictions of the HER volcano plot and the d‐band theory: Ni and Pt's activities were accelerated by compression, while Cu's activity was accelerated by tension. By isolating the elastic strain effect from the ligand effect, this study provides a greater insight into the role of elastic strain in controlling electrocatalytic activity.

Structural changes in amorphous Li_xSi during the initial lithiation of single crystal Si to amorphous Li_xSi were investigated as a function of Li flux and total charge using solid-state ⁷Li nuclear magnetic resonance (NMR) spectroscopy. Single crystal (100) Si wafers were used as a model system in this study, as the Li flux could be precisely controlled and uniformly distributed across the surface of the wafer. It was observed that peak shifts in solid-state ⁷Li NMR spectra varied as a function of the Li flux during the initial lithiation and stayed constant for samples of the same Li flux regardless of the total charge applied to the electrodes. We conclude from these results that the Li concentration in Li_xSi stays constant for a given flux regardless of the total coulombic charge applied to the electrode. The results from this study help better understand the kinetics, the reaction mechanisms, and the kinetic modeling of chemical reactions at the reaction front during the initial lithiation of Si (100).

We report real-time average stress measurements on composite silicon electrodes made with two different binders – viz. Carboxymethyl cellulose (CMC) and Polyvinylidene fluoride (PVDF) – during electrochemical lithiation and delithiation. During galvanostatic lithiation at very slow rates, the stress in a CMC-based electrode becomes compressive and increases to 70 MPa, where it reaches a plateau and increases slowly thereafter with capacity. The PVDF-based electrode exhibits similar behavior, although with lower peak compressive stress of about 12 MPa. These initial experiments indicate that the stress evolution in a Si composite electrode depends strongly on the mechanical properties of the binder. Stress data obtained from a series of lithiation/delithiation cycles suggests plasticity induced irreversible shape changes in contacting Si particles, and as a result, the stress response of the system during any given lithiation/delithiation cycle depends on the cycling history of the electrode. While these results constitute the first in situ stress measurements on composite Si electrodes during electrochemical cycling, the diagnostic technique described herein can be used to assess the mechanical response of a composite electrode made with other active material/binder combinations.

Analysis of lithiation and delithiation kinetics in pulse-laser-deposited crystalline thin-film silicon (Si) electrodes is presented. Data from open-circuit relaxation experiments are used in conjunction with a model based on Tafel kinetics and double-layer capacitance to estimate the apparent transfer coefficients (α_a, α_c) and exchange current density to capacitance ratio (i₀/C_dl) for lithiation and delithiation reactions in a lithiated silicon (Li_xSi) system. Parameters estimated from data sets obtained during first-cycle amorphization of crystalline Si, as well as from cycled crystalline Si and amorphous Si thin-film electrodes do not show much variation, indicating that they are intrinsic to lithiation/delithiation in Si. A methodology to estimate the side-reaction rate on a well-cycled electrode and its role in the evolution of the open-circuit potential of the Li_xSi system are discussed. We conclude that the large potential offset between lithiation and delithiation reactions at any given state of charge is partially caused by a large kinetic resistance (i.e., small i₀). Using the estimated parameters, the model is shown to predict successfully the behavior of the system under galvanostatic lithiation and delithiation.