SINS of The SEI Layer: On The Origins of Electrode Passivity of Li-Metal Anode

Li-ion and Li-metal batteries performance limitations are associated with physico-chemical processes at electrode/electrolyte interfaces, which lead to high impedance, electrochemical instability, and inhomogeneous Li plating and stripping. In fact, battery performance is largely determined by thermodynamic, kinetic, and mechanical properties of such electrochemical interfaces and interphases. These electrochemical systems tend to operate far away from equilibrium. A thin passive film, the so-called solid electrolyte interphase (SEI) layer, that forms at the electrode/electrolyte interface during battery assembly/formation, which gradually reforms during its operation is critical for its basic function and lifetime.<br/>However, our overall understanding of heterogeneous ionic interfaces and interphases is still very limited due to two main reasons. First, characterizing such interfaces and interphases in their native environment is extremely challenging, as they are buried between two dissimilar materials. Second, the interfaces and interphases have complex structure and chemistry, and can even evolve by chemical inter-diffusion, lattice strain, defects, and space charge effects which lead to a variety of chemical reactions across multiple spatial and temporal scales. Interface evolution is further propelled by the large amount of charge and mass transfer between electrodes in a battery over its lifetime (generally leading to degradation, performance loss, and eventual battery failure.<br/>In this work, we overview and exploit the nanoscale spatial resolution, chemical selectivity, and surface sensitivity of near-field infrared nanospectroscopy to characterize electrode/liquid and solid electrolyte interfaces. Near-field infrared measurements in combination with standard surface characterization tools reveal that intrinsic molecular, structural, and chemical heterogeneities at the interface. This work provides a unique insight into the mechanisms of early-stage interphase formation at electrochemically active buried interfaces, and an experimental diagnostic means to aid in the development of methods to control local nanoscale variations in electrolyte chemistry, structure, and ionic conductivity at the surface of the electrode.