Probing the Stability of Lithium Carbonate in the Lithium-Metal Solid Electrolyte Interphase

Abstract

The chemical composition and structure of the solid electrolyte interphase (SEI) are two of the key factors that determine the reversibility of lithium-metal (Li) anodes for next-generation batteries. As a result, much of the research aimed at enabling practical Li-metal batteries emphasizes tuning SEI composition, either via electrolyte formulation1–5 or synthesis of artificial SEIs.6–8 Ideally, the lithium SEI should minimize parasitic side reactions by effectively passivating Li while also promoting facile conduction of lithium ions (Li+). To do this, SEI materials must have high (electro)chemical stability, be ionically conductive, and be sufficiently mechanically robust to accommodate substantial volume changes. However, studying these properties in bulk-scale materials often yields values that diverge by orders of magnitude from those observed in SEIs. For example, typical SEI ionic conductivities lie in the range of 10-7-10-9 S cm-1, yet bulk ionic conductivity measurements of relevant materials such as lithium carbonate, lithium fluoride, and lithium oxide ranges from 10-18 and 10-10 S cm-1.9 Our group has developed a technique to directly study these materials at realistic length scales by synthesizing model interphases through the reaction of gases with Li.10,11 Our previous work on Li2O and LiF revealed that Li2O is a better Li+ conductor than LiF (~1 x 10-9 S cm-1 vs ~5.2 x 10-10 S cm-1),11 and that these species’ chemical stability varies substantially in different electrolytes.12 One of the remaining key SEI materials is lithium carbonate, which has been proposed to act as a metastable phase in the outer portion of the SEI.13 In this work, we have developed a technique to synthesize Li2CO3 films via sequential reaction of oxygen and carbon dioxide with clean lithium surfaces. Using scanning electron microscopy and air-exposure tests, we can confirm that these films are conformal and generally pinhole-free. Titration gas chromatography (TGC)14 was used to quantify relative proportions of lithium carbonate, metallic lithium, and lithium carbide, and X-ray photoelectron spectroscopy (XPS) offers insights into how composition changes across the depth of the film. These films were then used as a platform to further investigate the reactivity of Li2CO3 with different electrolytes, comparing carbonates versus ethers and varying the lithium salt used. Electrochemical impedance spectroscopy (EIS) offers insights into the evolution of transport properties at these interphases, while electrolyte soak tests coupled with gas chromatography of gas-phase products and TGC of solid-phase products can track their chemical evolution. 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