The ability to utilize Li metal as the anode in Li-ion or Li batteries has been a long-sought target since the development of modern battery technology because of its high capacity compared to graphite-based anodes (3860 mAh/g_Li vs. 372 mAh/g_graphite), thus offering the prospect of significantly higher cell energy densities. However, Li faces several drastic challenges that have impeded its successful use as an anode so far. These include:¹ (1) Large volume expansion upon Li stripping and plating, leading to rapid mechanical degradation; (2) Poor Coulombic (round-trip) efficiencies during cycling, which leads to loss of active Li (formation of so-called “dead Li”); and (3) Formation of dendritic structures upon Li deposition, which grow catastrophically and ultimately short-circuit the cell. Researchers have attempted to address these issues by developing solid electrolyte materials of sufficient mechanical strength to physically block dendrites, but such composites generally have high ionic resistances² and poor performance at high powers³ due to the minimum thicknesses required for fabricating and processing without breakage.

Recently, attention has shifted towards the introduction of “designer” material layers on Li through pre-formed SEI structures.^4-6 Such a motif can be accomplished, for example, by introducing clean Li surfaces to donor molecules or additives that contain desirable constituents, such as fluoride ligands, that will presumably be incorporated upon decomposition at the Li surface. However, one major challenge is a lack of fundamental understanding of how such molecules decompose when in contact with a Li surface, either under dry conditions or in the presence of electrolyte. Consequently, there remains little understanding regarding how to intentionally build SEI layers with targeted compositions, morphological and electronic properties, and how these properties translate into electrochemical metrics such as Coulombic efficiency and cycle life.

In this work, we present our recent studies of the decomposition reactions of oxide- and fluoride-containing gases on Li surfaces for artificial SEI formation. We first demonstrate controllable growth (10 – 100’s of nm thickness) of targeted ionic materials such as Li₂O and LiF which is achieved through judicious selection of the reactant gas and reaction conditions. In gases with multiple potential SEI-forming constituents, we find that reaction kinetics and the decomposition pathway determine which elements are ultimately imparted to an SEI, thereby introducing an ability to rationally tune the SEI by tuning the gas structure and chemistry. We report X-ray photoelectron (XPS) with depth-profiling and X-ray diffraction (XRD) analysis of films to describe their chemical composition and crystallinity. We also employ several methodologies to characterize the electronic and electrochemical behavior of such by-design films, including electrochemical impedance, Li-Li cycling, Li-Cu plating/stripping measurements, and constant-current discharge to failure to measure dendrite propagation times, and compare their behavior to bare Li. Special emphasis will be placed on the mechanistic origin of electrochemical properties, which are derived from Li⁺ transport through film grain boundaries, and on the failure modes of such films during cycling.

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3. Bachman, J. C.; Muy, S.; Grimaud, A.; Chang, H. H.; Pour, N.; Lux, S. F.; Paschos, O.; Maglia, F.; Lupart, S.; Lamp, P.; Giordano, L.; Shao-Horn, Y., Inorganic Solid-State Electrolytes for Lithium Batteries: Mechanisms and Properties Governing Ion Conduction. Chem Rev 2016, 116 (1), 140-162.
4. Li, Y.; Sun, Y.; Pei, A.; Chen, K.; Vailionis, A.; Li, Y.; Zheng, G.; Sun, J.; Cui, Y., Robust Pinhole-free Li3N Solid Electrolyte Grown from Molten Lithium. 2018, 4 (1), 97-104.
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