In this work we explore how an electrolyte additive (fluorinated ethylene carbonate – FEC) mediates the thickness and composition of the solid electrolyte interphase formed over a silicon anode in situ as a function of state-of-charge and cycle. We show the FEC condenses on the surface at open circuit voltage then is reduced to C-O containing polymeric species around 0.9 V (vs. Li/Li⁺). The resulting film is about 50 Å thick. Upon lithiation the SEI thickens to 70 Å and becomes more organic-like. With delithiation the SEI thins by 13 Å and becomes more inorganic in nature, consistent with the formation of LiF. This thickening/thinning is reversible with cycling and shows the SEI is a dynamic structure. We compare the SEI chemistry and thickness to 280 Å thick SEI layers produced without FEC and provide a mechanism for SEI formation using FEC additives.

The transport of protons is critical in a variety of bio- and electro-chemical processes and technologies. The Grotthuss mechanism is considered to be the most efficient proton transport mechanism, generally implying a transfer of protons between ‘chains’ of host molecules via elementary reactions within the hydrogen bonds. Although Grotthuss proposed this concept more than 200 years ago, only indirect experimental evidence of the mechanism has been observed. Here we report the first experimental observation of proton transfer between the molecules in pure and 85% aqueous phosphoric acid. Employing dielectric spectroscopy, quasielastic neutron, and light scattering, and ab initio molecular dynamic simulations we determined that protons move by surprisingly short jumps of only ~0.5–0.7 Å, much smaller than the typical ion jump length in ionic liquids. Our analysis confirms the existence of correlations in these proton jumps. However, these correlations actually reduce the conductivity, in contrast to a desirable enhancement, as is usually assumed by a Grotthuss mechanism. Furthermore, our analysis suggests that the expected Grotthuss-like enhancement of conductivity cannot be realized in bulk liquids where ionic correlations always decrease conductivity.

Deposition of Li is a major safety concern existing in Li-ion secondary batteries. Here we perform the first in situ high spatial resolution measurement coupled with real-time quantitative electrochemistry to characterize SEI formation on gold using a standard battery electrolyte. We demonstrate that a dendritic SEI forms prior to Li deposition and that it remains on the surface after Li electrodissolution.

Ionic liquids (ILs) and deep eutectic solvents (DESs) have tremendous potential for reactive capture of CO ₂ , due to their highly properties, including a wide electrochemical stability window, low volatility, and high CO ₂ solubility.

Lithium phosphorus oxynitride (Lipon) is an amorphous solid-state electrolyte that can completely suppress Li penetration from the anode to the cathode, commonly referred to as dendrites. The key to the Lipon performance is thought to be its homogenous and pore free morphology. To test this, we present a modified thin film battery configuration with a lithium cobalt oxide cathode, a Lipon electrolyte, and a top layer with a copper current collector and an artificial Lipon-Lipon interface parallel to the cathode. Upon electrochemical cycling Li metal rapidly deposits at the edge of this Cu current collector and then proceeds to plate along the 2D Lipon-Lipon interface. As the Li is confined to this 2D plane, it confirms the ability of Lipon to suppress Li penetration. It also demonstrates that the homogenous interface-free morphology of Lipon is key to its performance.