Single-Component Solid Electrolyte Interphases on Lithium
Authors: Gallant Betar M. , He Mingfu , Guo Rui 
Published: February 2020

The ability to utilize Li metal anodes in rechargeable batteries would be an enabling breakthrough in the quest to increase energy density and meet current aggressive performance targets (> 350 Wh/kg) for electric vehicles. However, Li anodes face several persistent challenges:1
,2 (1) Large volume expansion upon cycling, leading to rapid mechanical degradation; (2) Poor Coulombic efficiencies during cycling, which leads to loss of active Li and formation of so-called “dead Li”; and (3) Evolution of dendritic structures upon Li deposition, which grow rapidly and ultimately short-circuit the cell. The origins of these problems can be traced back to the chemical, ionic, and mechanical properties of the solid electrolyte interphase (SEI)3- consisting of ionically conductive, physically blocking solid inorganic and organic phases derived from the reaction of Li with the electrolyte - and the electrochemical processes that break down this layer. After decades of study, however, the native SEI, which is highly chemically nonuniform and nanoscopically thin (< 100 nm), remains notoriously difficult to experimentally probe. Consequently, there remains a need for improved understanding of which Li-derived phases are desirable or undesirable in the SEI, their precise roles, and how to manipulate or modify the SEI properties for improved function and stability.

In this talk, we present our recent studies of single-component artificial interfaces on Li, which we have conducted to gain deeper insights into the possible roles of common interfacial constituents found in nearly all Li SEI.4,5We have developed synthetic approaches based on gas-phase reactions with oxide- and fluoride-donating reactants that permit introduction of single phases with high spatial uniformity. We first show results of controllable growth (10 – 100’s of nm thickness) of targeted ionic materials Li2O and LiF, which is achieved through judicious selection of the reactant gas composition and reaction conditions. The formed layers are conformal, compact, polycrystalline, and chemically stabile in carbonate electrolytes, and thus serve as model all-ionic interphases useful for further qualitative and quantitative study. Following the establishment of these “ionically enriched” Li anodes, we subject these interphases to electrochemical measurements including electrochemical impedance, galvanostatic cycling (relevant to practical battery operating conditions), as well as softer electrochemical techniques such as chronoamperometry, which allow the minimal overpotentials and current densities driving deposition through the layers to be determined. We find evidence that there exist maximum sustainable overpotentials and current densities in the all-ionic layers, above which degradation mechanisms are activated within the films, leading to breakage. Using defect and transport modeling applied to the electrochemical results, we are able to quantify transport parameters including Li+vacancy carrier concentrations and diffusion coefficients, and compare these to the bulk materials. Our results show that interfacially-formed Li2O and LiF have unique properties when they are formed on Li metal. Current understanding of SEI behavior, which is largely based on knowledge of bulk properties of Li2O and LiF formed under drastically different thermochemical conditions, should be updated to reflect the unique structural, chemical and electrochemical properties occurring at interphases derived directly from Li.

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