Direct, Operando Observation of the Bilayer Solid Electrolyte Interphase Structure: Electrolyte Reduction on a Non-Intercalating Electrode
Christopher H. Lee, Joseph A. Dura, Amy LeBar, Steven C. DeCaluwe
The solid electrolyte interphase remains a central challenge to lithium-ion battery durability, in part due to poor understanding of the basic chemistry responsible for its formation and evolution. In this study, the SEI on a non-intercalating, thin-film tungsten anode is measured in operando by neutron reflectometry and quartz crystal microbalance. A dual-layer SEI structure is observed, with a 3.7 nm thick inner layer and a 15.4 nm thick outer layer. Such structures have been proposed in literature but have not been definitively observed via neutron reflectometry. The SEI mass per area was 1207.2 ng-cm2, and QCM with simultaneous electrochemistry provides insight into the formation dynamics of the two-layer SEI during a negative-going voltage sweep and its evolution over multiple cycles. Monte Carlo simulations identify SEI chemical compositions consistent with the combined measurements. The results are consistent with a primarily inorganic, dense inner layer and a primarily organic, porous outer layer, directly confirming structures proposed in the literature. Futher refinement of techniques presented herein, coupled with complementary measurements and simulations, can give direct, quantitative insight into SEI formation and evolution. This, in turn, will enable the scientifically-guided design of durable, conductive SEI layers for Li-ion batteries for a range of applications.