PNNL

Abstract

Accurate understanding of the chemistry of solid-electrolyte-interphase (SEI) and the origin and evolution of its key components constitutes a key to developing new electrolytes for high-energy batteries based on lithium metal (Li0) anode. SEI on Li0 is generally believed to be formed by solid products resulting from the reactions between Li0 and electrolyte components (solvents, anion and additives). However, our new study shows this is not the whole story. Through synchrotron-based X-ray diffraction and pair distribution function analysis, we reveal a much more convoluted formation mechanism of SEI, which receives contributions not only from Li0 but also from cathode, moisture, and native surface species on Li0, with highly dynamic nature during charge-discharge cycling. Using isotope-labeling, we traced the origin of LiH to both the electrolyte itself and the moisture dissolved in it, and an unreported new source, the native surface species (LiOH) on pristine Li0. As the universal native species on Li0, LiOH is identified to be metastable and further amalgamates into large crystals during the cycling of Cu||LiNi0.8Mn0.1Co0.1O2 (NMC811) cell, which could be an additional reason why anodeless cells degrade rapidly. Alternatively, LiOH can react with Li0 via separate pathways to form LiH and Li2O as in the case of Li||NMC811 cell. While the desired anion reduction on Li0 for LiF-rich SEI is typically found in the concentrated electrolytes, we found it can also be realized in diluted electrolyte via the crosstalk between Li0 anode and NMC811 cathode, opening a new approach for low-cost electrolyte development. These discoveries delineate a rather convoluted mechanism for a series of inorganic interphasial components that are intrinsically correlated.