PNNL

Abstract

Designing cathode materials which are chemically and structurally stable under repetitive ion insertion and extraction has been a bottleneck in the development of multivalent batteries. The cycling stability of traditional metal oxide-based cathode is challenged by sluggish diffusion of multivalent cations in both the bulk and interfacial regimes, including the cathode electrolyte interphase layer (CEI). The understanding of interfacial reactions at the cathode-electrolyte interface, especially those induced by surface defects, is a critical design parameter for cathode materials as well as the electrolyte. In this study, we employed multimodal analysis, including in situ and ex situ X-ray photoelectron (XPS), high resolution transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) to examine the surface reactions and subsequent structural and chemical evolutions of the CEI on high voltage magnesium vanadate (MgV2O4, demoted as MVO) spinel cathode that occurred with repeating Mg2+ insertion/extraction process. The result showed that the surface defects of the MVO cathode drive bis(trifluoromethanesulfonyl)imide (TFSI-) anion decomposition leading to CEI layer formation, which could inhibit Mg2+ ion transfer processes. Accompanying this chemical degradation, the MVO cathode also undergoes pulverization and formed clusters of nanosized particles that likely improved the cycling ability by rendering new intercalation sites and decreasing the diffusion pathway of the cations. This study demonstrates that surface defects control and particle engineering are critical design parameter for high-performance cathodes for multivalent batteries.