In Situ Measurement of the Plane-Strain Modulus of the Solid Electrolyte Interphase on Lithium-Metal Anodes in Ionic Liquid Electrolytes

Abstract

We present an experimental approach for in situ measurement of elastic modulus of the solid electrolyte interphase (SEI), which is formed from reactions between a lithium thin-film [on a polydimethylsiloxane (PDMS) substrate] and a room-temperature ionic liquid (RTIL) electrolyte. The SEI forms under a state of compressive stress, which causes buckling of the sample surface. In situ atomic force microscopy is used to measure the dominant wavelength of the wrinkled surface topography. A mechanics analysis of strain-induced elastic buckling instability of a stiff thin film on a soft substrate is used to determine the plane strain modulus of the SEI from the measured wavelength. The measurements are performed for three RTIL electrolytes: 1-butyl 1-methylpiperidinium bis(trifluoromethylsulfonyl)imide (P14 TFSI) without any lithium salt, 1.0 M lithium bis(trifluoromethylsulfonyl)imide (Li TFSI) in P14 TFSI, and 1.0 M lithium bis(fluorosulfonyl)imide (Li FSI) in P14 TFSI to investigate the influence of lithium salts on the plane strain modulus of the SEI. The measurements yield plane-strain moduli of approximately 1.3 GPa for no-salt P14 TFSI and approximately 1.6 GPa for 1.0 M Li TFSI in P14 TFSI and 1.0 M Li FSI in P14 TFSI. The experimental technique presented here eliminates some of the uncertainties associated with traditional SEI mechanical characterization approaches and offers a platform to engineer an SEI with desired mechanical properties by approaches that include altering the electrolyte composition.