RESUMEN
Formation and evolution of the microscopic solid electrolyte interphase (SEI) at the Mg electrolyte/electrode interface are less reported and need to be completely understood to overcome the compatibility challenges at the Mg anode-electrolyte. In this paper, SEI evolution at the Mg electrolyte/electrode interface is investigated via an in situ electrochemical quartz crystal microbalance with dissipation mode (EQCM-D), electrochemical impedance spectroscopy (EIS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), and Fourier transform infrared spectrometry (FTIR). Results reveal remarkably different interfacial evolutions for the two Mg electrolyte systems that are studied, a non-halogen Mg(TFSI)2 electrolyte in THF with DMA as a cosolvent (nhMg-DMA electrolyte) versus a halogen-containing all-phenyl complex (APC) electrolyte. The nhMg-DMA electrolyte reports a minuscule SEI formation along with a significant Coulomb loss at the initial electrochemical cycles owing to an electrolyte reconstruction process. Interestingly, a more complicated SEI growth is observed at the later electrochemical cycles accompanied by an improved reversible Mg deposition attributed to the newly formed coordination environment with Mg2+ and ultimately leads to a more homogeneous morphology for the electrochemically deposited Mg0, which maintains a MgF2-rich interface. In contrast, the APC electrolyte shows an extensive SEI formation at its initial electrochemical cycles, followed by a SEI dissolution process upon electrochemical cycling accompanied by an improved coulombic efficiency with trace water and chloride species removed. Therefore, it leads to SEI stabilization progression upon further electrochemical cycling, resulting in elevated charge transport kinetics and superior purity of the electrochemically deposited Mg0. These outstanding findings augment the understanding of the SEI formation and evolution on the Mg interface and pave a way for a future Mg-ion battery design.
RESUMEN
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.