In Situ Probing of Mass Exchange at the Solid Electrolyte Interphase in Aqueous and Nonaqueous Zn Electrolytes with EQCM-D.

Multivalent chemistry provides intriguing benefits of developing beyond lithium ion energy storage technologies and has drawn extensive research interests. Among the multivalent candidates, metallic zinc anodes offer an attractive high volumetric capacity at a low cost for designing the secondary ion batteries. However, the interfacial mass exchange at the Zn electrolyte/anode boundary is complicated. The least understood solid electrolyte interphase (SEI) occurs simultaneously with the reversible metal deposition, and its dynamic progression is unclear and difficult to capture. One major challenge to investigate such a dynamic interface is the lack of in situ analytical methods that offer direct mass transport information to reproduce the realistic battery operating conditions in an air-sensitive, nonaqueous electrolyte environment with a high iR drop. Work reported here reveals an in-depth analysis of the complex and dynamic SEI at the Zn electrolyte/electrode interface utilizing a multiharmonic quartz crystal microbalance with a dissipation method combined with the spectroscopic analysis. Key differences are observed for the SEI formation in the nonaqueous Zn(TFSI)2 electrolyte in contrast to the aqueous ZnCl2 electrolyte for reversible Zn deposition. A large disproportional loss of coulombs relative to the gravimetric mass change is prominently observed at the initial electrochemical cycles in the nonaqueous Zn electrolyte, and results suggest an in situ formation of an ionically permeable SEI layer that is compositionally featured with a rich content of organic S and N components. Further overtone-dependent dissipation analysis implies the changes in viscoelasticity at the electrode interface during the early SEI formation in the nonaqueous Zn(TFSI)2 electrolyte.