Gustavo Hobold1 Betar Gallant1

1, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

Despite possessing physical properties that justify their use in high voltage, high power aprotic battery chemistries, carbonate-based electrolytes and their resulting solid electrolyte interphases (SEI) do not generally enable morphologically homogeneous plating and stripping to and from metal electrodes. Instead, due to interphase breakdown, freshly plated metal is continuously exposed to the electrolyte. High electrode-electrolyte reactivity causes loss of coulombic efficiency and promotes dendrite growth, ultimately leading to catastrophic cell failure. Efforts to circumvent this issue have often focused on the ex-situ design of an artificial inert SEI or on engineering customized electrolyte systems that are hypothesized to form stable interphases in-situ. Still, due to the difficulty in probing the SEI at the molecular scale, little is known about the dynamics of its formation and breakdown during plating and stripping, and the products thereof. Here we investigate the gas phase products resulting from lithium plating and stripping under operando conditions using a custom electrochemical cell coupled to a gas chromatograph. Under electrochemical activity, we quantify the evolution of several C1-C2 species, as well as CO, CO2 and H2, that are dynamically formed due to electrolyte decomposition and SEI formation and breakdown. The operando nature of our experiments enables the identification of gases evolved through pathways that have rate-limiting electron-transfer steps, as well as gases that have chemical reactions as rate-limiting steps. Moreover, we show that SEI transformations, such as the chemical conversion of interfacial Li2CO3 into Li2O, release detectable gas signatures, which are then correlated with interfacial composition measured by X-ray photoelectron spectroscopy. Because some species are formed exclusively by pathways involving electron transfer, we correlate gas evolution with plating-stripping coulombic efficiency, and hence capacity loss. By varying galvanostatic conditions, we also observe rate-dependent gas evolution, which we link to surface morphology using ex-situ scanning electron microscopy. We then investigate electrolyte systems that are known to enable morphologically homogeneous plating-stripping, in which gas evolution is expected to be suppressed due to the formation of a likely stable SEI. Finally, we suggest that operando gas analysis can be effective in identifying species and their formation mechanisms that result from parasitic electrode-electrolyte reactions, the evolution of which needs to be suppressed to achieve high cycling stability.