Real-Time Stress Measurements in Germanium Thin Film Electrodes during Electrochemical Lithiation/Delithiation Cycling

Abstract

An in situ study of stress evolution and mechanical behavior of germanium as a lithium-ion battery electrode material is presented. Thin films of germanium are cycled in a half-cell configuration with lithium metal foil as counter/reference electrode, with 1M LiPF 6 in ethylene carbonate, diethyl carbonate, dimethyl carbonate solution (1:1:1, wt%) as electrolyte. Real-time stress evolution in the germanium thin-film electrodes during electrochemical lithiation/ delithiation is measured by monitoring the substrate curvature using the multi-beam optical sensing method. Upon lithiation a-Ge undergoes extensive plastic deformation, with a peak compressive stress reaching as high as −0.76 ± 0.05 GPa (mean ± standard deviation). The compressive stress decreases with lithium concentration reaching a value of approximately −0.3 GPa at the end of lithiation. Upon delithiation the stress quickly became tensile and follows a trend that mirrors the behavior on compressive side; the average peak tensile stress of the lithiated Ge samples was approximately 0.83 GPa. The peak tensile stress data along with the SEM analysis was used to estimate a lower bound fracture resistance of lithiated Ge, which is approximately 5.3 J/m 2 . It was also observed that the lithiated Ge is rate sensitive, i.e., stress depends on how fast or slow the charging is carried out.

Comment on “In Situ Measurements of Stress-Potential Coupling in Lithiated Silicon” [J. Electrochem. Soc., 157, A1253 (2010)], “On Plastic Deformation and Fracture in Si Films during Electrochemical Lithiation/Delithiation Cycling” [J. Electrochem. Soc., 160, A1885 (2013)], and “Real-Time Stress Measurements in Germanium Thin Film Electrodes during Electrochemical Lithiation/Delithiation Cycling” [J. Electrochem. Soc., 162, A2840 (2015)]

“…1) and that shown in Ref. 3 ( Fig. 1): i) In the first two cases 1,2 the solution level is higher than the silica plate so the reflecting surface is submerged in solution, while in Ref.…”

“…1): i) In the first two cases 1,2 the solution level is higher than the silica plate so the reflecting surface is submerged in solution, while in Ref. 3 the top face of the silica wafer is out of the liquid phase and is in contact with a gas phase. ii) In the experimental arrangement shown in Ref.…”

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

“…Due to an equal biaxial stress state of the bilayer electrode during electrochemical process, [18,[31][32][33][42][43][44] the stress field along the thickness of the bilayer electrode can then be given by the constitutive equation, as follows:…”

“…Guduru and collaborators have made a great contribution in this regard. These investigators applied MOSS to measure the deformation and stress of various electrodes during electrochemical lithiation and delithiation cycles, such as those of Si, [31] graphite, [32] Ge, [33] Sn, [34] and Li 1 + x Mn 2 O 4 . [35] The values of compressive or tensile stresses in different electrodes or of various thicknesses exhibited great differences and showed different trends with capacity.…”

“…[35] The values of compressive or tensile stresses in different electrodes or of various thicknesses exhibited great differences and showed different trends with capacity. [13,[31][32][33][34][35] 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. And in this method the calculated stress by Stoney equation ignores bending stress and, more importantly, ignores the already confirmed concentration-dependent elastic modulus of the active material.…”