(Invited) In Situ Measurement of Mechanical Properties of Solid Electrolyte Interphase (SEI) Layers

With increasing demand for light-weight, high energy density energy storage, lithium metal electrodes are being actively revisited decades after its abandonment in the 1970s. Metallic lithium can play the role of host-free electrode, and thus has much higher charge capacity compared to traditional graphite-based electrodes. The major challenge with the lithium electrode is the generation of dendrites, resulting in internal short-circuits and poor cell cyclability. Several approaches, including electrolyte engineering and artificial solid electrolyte interphase (SEI) formation, are being pursued to resolve lithium dendrite formation. Recent theoretical studies of lithium metal electrodes suggest that surface films with high elastic modulus suppress dendrite growth. Inhibition of lithium dendrite with ionic liquid electrolytes, which are known to form inorganic SEI, support this theoretical suggestion. This background provides the motivation to develop an experimental method to measure elastic moduli, stress-strain curve and fracture toughness of SEI formed with various electrolytes. In this work, we report results of two experimental methods to measure mechanical properties of SEI: (i) micro-scale membrane bulge tests and (ii) strain induced elastic buckling instability (SIEBI). Lithium thin film electrodes (~50 nm) were prepared on a polydimethylsiloxane (PDMS) substrate by thermal evaporation and converted to SEI through chemical and electrochemical reactions. Various organic and ionic liquid electrolytes were used to understand compositional influence on the mechanical properties of SEI. SEI layers either remain flat or buckle depending on the elastic modulus and the residual stress state, which are governed by the electrolyte composition. For the flat SEI layer case, rectangular free-standing PDMS/SEI membrane were fabricated and subjected to bulge testing in inert atmosphere. The maximum deflection at each pressure was measured by an atomic force microscope (AFM) to get the pressure-deflection curve, which was analyzed to obtain the elastic modulus and stress-strain curve of the SEI layers. For the buckled SEI layer case, the elastic modulus of SEI layer was calculated from the wavelength of the buckling pattern. These two approaches allow us to study the role of electrolyte composition on the mechanical properties of SEI layers. It is be possible to extend these methods to study the inelastic behavior and fracture properties of SEI as well.