Back to the basics: Advanced understanding of the as-defined solid electrolyte interphase on lithium metal electrodes

Abstract

The design and control of the solid electrolyte interphase (SEI) is essential for the implementation of rechargeable lithium metal based batteries and has been a major research theme of lithium metal electrodes for over 50 years. However, research on an as-defined SEI is challenging due to the inevitable presence of a native passivation layer (NPL), commonly contaminating an in situ formed SEI and its characteristics. Herein we present an original study on an uncontaminated SEI being an exclusive reaction product of lithium and a liquid aprotic electrolyte, highlighting the largely negative influence of the NPL on different SEI characteristics and functional additive evaluation. A preformed as-defined SEI (pSEI) was generated by an in-house designed advanced pre-treatment method, ensuring high reproducibility and exclusion of the NPL by cutting and pressing lithium metal in solution/electrolyte. The implementation of a pSEI allows for a systematic study and an unclouded comparison of the two common functional additives vinylene carbonate (VC) and fluoroethylene carbonate (FEC) in an equal molar setting. The introduction of an as-defined SEI allowed to clearly distinguish between enhancing influence on the SEI characteristics and full cell performance by FEC and the largely negative effect of VC on different SEI characteristics.