Measurements of the Phase and Stress Evolution during Initial Lithiation of Sn Electrodes

Abstract

We have measured the phase kinetics and stress evolution during potentiostatic lithiation of Sn thin film layers to form the first lithiated phase (Li 2 Sn 5 ). Real-time measurements of the wafer curvature are used to determine the stress evolution during lithiation; the corresponding layer structure is determined from the measured charge and confirmed with cross-sectional focused-ion beam (FIB) images and X-ray diffraction (XRD). Prior to new phase formation, a 20-hour conditioning step is used to form a solid-electrolyte interphase (SEI) layer. The Stress in the SEI and Sn layers are determined by stress measurements carried on different thicknesses of Sn film. When the potential is lowered to initiate Li 2 Sn 5 formation, both the electrochemical current and stress exhibit a complex transient behavior as Li 2 Sn 5 phase nucleates and grows. The stress evolution in this regime is attributed to rate-dependent plasticity in the Sn layer. At longer times, as the new phase propagates into Sn, a steady-state is reached that enables us to determine the stress in the Li 2 Sn 5 layer. The phase evolution in the steady-state regime is analyzed with a kinetic model that enables us to estimate the Li diffusivity in Li 2 Sn 5 and the reaction rate coefficient controlling the phase transformation.

Numerical Solution of Moving Phase Boundary and Diffusion-Induced Stress of Sn Anode in the Lithium-Ion Battery

“…In the previous work, 2 we reported that the low strain-rate yield stress of Li 2 Sn 5 is around −29 MPa. We also observed the tensile contribution with a value of 8.8 MPa-μm from the SEI at the anode surface.…”

“…Further details of the sample fabrication have been described in the previous report. 2 The cells were built and operated in an Ar-filled glove box. In order to distinguish the charge consumed by SEI formation and lithiation as much as possible, the anodes were first processed to grow a SEI layer at 0.8 V vs. Li/Li + for 20 hours before growth of the lithiated Sn phase.…”

“…The C s value was determined experimentally by performing a potentiostatic intermittent titration technique (PITT) experiment on a fully transformed Li 2 Sn 5 layer; details are provided in the previous publication. 2 The parameters used in the simulation are provided in Table I, and the results of fitting parameters are presented in Table II.…”

Modified Stoney Model and Optimization of Electrode Structure Based on Stress Characteristics

“…Therefore, increases linearly with β . However, as far as currently known, experimentally measured deformation changes nonlinearly with Li concentration ,,. Furthermore, the variable nature of the elastic modulus of the active layer during electrochemical processes has been experimentally proven ,.…”

“…These investigators applied MOSS to measure the deformation and stress of various electrodes during electrochemical lithiation and delithiation cycles, such as those of Si, graphite, Ge, Sn, and Li 1+ x Mn 2 O 4 . The values of compressive or tensile stresses in different electrodes or of various thicknesses exhibited great differences and showed different trends with capacity ,. However, this technology requires a thick wafers about 500∼1000 um as a measurement basis, therefor fails to characterize the stress in a current collector which is also important.…”