Operando ORP-EIS for Monitoring SEI Formation of Anode-Free Li Metal Batteries

The solid electrolyte interphase (SEI) can significantly improve the low Coulombic Efficiency and poor cycling performance of anode-free Li metal batteries (AFLMBs). However, the sensitive nature of the SEI layer, its restricted environment, and the complex working conditions of the battery system complicate its investigation under operational conditions. We apply operando odd random phase electrochemical impedance spectroscopy (ORP-EIS) to monitor the formation of the SEI layer for AFLMBs. Operando ORP-EIS is measured for the Cu substrate during cathodic polarization from the open-circuit voltage to −0.4 V vs Li/Li + in 1 M LiPF 6 in EC/DMC (1:1 v/v), providing qualitative information regarding the reaction resistances during SEI formation. To further study the properties of the SEI layer and support the operando ORP-EIS findings, we employ surface analysis techniques, including field-emission gun scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). We observe that the formation of different compounds on the anode (negative electrode) as a function of applied charging potential influences the SEI properties. This nondestructive operando ORP-EIS technique provides an opportunity to obtain realistic information about the SEI layer and measure charge transfer resistance of occurring reactions. This technique facilitates the development of AFLMBs.