Revealing the Functionality of Li2CO3 in the Li-Metal SEI Via Study of Gas-Reacted Li Films
Authors: Steinberg Katherine , Gallant Betar M. 
Published: August 2024

The solid electrolyte interphase (SEI) governs transport and reactivity at lithium interfaces, so its structure and composition are essential factors in improving the cyclability of next-generation lithium-metal anodes (1). An ideal SEI should passivate Li against continuous reactions with electrolyte while promoting facile transport of Li+ ions. However, achieving these properties is challenging, in part because our understanding of the relative desirability of different SEI materials is often based on qualitative relationships between characterization and cell performance; quantitative experimental conductivity (2, 3) and stability (4) measurements are limited. As a further complication, the highly reductive conditions (-3.04 V vs SHE) and the complex nanoscale structure of the SEI can lead to markedly different behavior in practical contexts compared to bulk environments (2, 5, 6). In previous work, our group developed techniques to synthesize single-component, nanoscale films of LiF and Li2O on Li, enabling direct measurements of their transport properties and reactivity in relevant electrolytes (6-8). Here, we turn to Li2CO3, which has a mixed reputation as an SEI material. While many papers assert its desirability based on high ionic conductivity (9-12), others raise concerns related to reductive instability (13-15).
In this work, we synthesized Li2CO3 films via sequential reactions of O2 and CO2 on polished lithium surfaces at slightly elevated temperature (175-200°C). Scanning electron microscopy (SEM) and air exposure tests showed that the prepared films are conformal, tens of nanometers thick, and relatively pinhole-free. Fourier transform infrared spectroscopy (FTIR) was used to confirm the speciation of the films, and titration-based quantification yielded insights into their composition. We found that the formation of Li2CO3 is associated with generation of Li2O and Li2C2, confirming that the reductive instability of Li2CO3 results in the evolution of a more reduced inner SEI layer at the Li | Li2CO3 interface. We also studied the stability of films at the SEI | electrolyte interface, performing electrolyte soak tests then assessing native SEI evolution using FTIR and titration-based quantification of LiF. We found that Li2CO3 is poorly-passivating in fluorinated electrolytes, leading to continuous formation of native SEI. However, in 1M LiClO4 PC electrolyte, the Li2CO3 film remains intact, enabling the use of electrochemical impedance spectroscopy (EIS) to study film transport properties. This analysis reveals an ionic conductivity of ~4-8 nS/cm, which is substantially greater than ionic conductivities previously measured in Li2O (~1 nS/cm) and LiF (~0.5 nS/cm) (6). Together, these results show that though Li2CO3 has some stability limitations, it could promote facile Li+ ion transport as a stable meso-SEI layer in less-fluorinated electrolytes.
This work was funded by the 2022-2023 ECS Toyota Young Investigator Fellowship award.

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