Developing Mechanistic Understanding of Zinc Plating Vs Zinc-Ion Intercalation in Chevrel-Phase Mo6Se8

Electrochemical ion intercalation is one charge-storage mechanism responsible for the operation of several battery chemistries such as rechargeable lithium (Li)-ion and zinc (Zn)-ion batteries. One possible failure mechanism for intercalation-based energy storage systems is uncontrolled metal electrodeposition (i.e., plating), where metal ions reduce on the ion-intercalation electrode surface instead of intercalating. Metal plating harms battery performance as the process consumes active material and can short-circuit the battery. The mechanistic interplay between electrochemical ion intercalation and metal plating is not yet well-understood. Elucidating a mechanism that describes the conditions necessary for metal to plate on ion-intercalation electrodes could facilitate development of suppression strategies.

In this work, we studied Zn plating in Zn-ion batteries wherein the interplay between electrochemical Zn²⁺ intercalation and Zn plating was measured in chevrel-phase Mo₆Se₈ cathodes. We demonstrate for the first time that Zn can plate on Mo₆Se₈ cathodes in Zn/Mo₆Se₈ cells. The occurrence of Zn plating on Mo₆Se₈ was studied through measurement of the nucleation overpotential, which appears as a minimum in the Zn/Mo₆Se₈ cell potential during galvanostatic discharge below 0 V. Variable-temperature (10 to 60 °C) and variable-current density (0.02 to 1 mA/cm²) measurements of the nucleation overpotential were conducted to elucidate a mechanism that describes the competition between electrochemical Zn²⁺ intercalation in Mo₆Se₈ and Zn plating on the electrode surface. The variable-temperature galvanostatic measurements in Zn/Mo₆Se­₈ cells show two unique electrochemical reaction mechanisms during Zn/Mo₆Se₈ discharge: (1) Zn²⁺ intercalation and Zn plating punctuated by the hydrogen evolution reaction (HER) in aqueous 1 M ZnSO₄ electrolyte, and (2) Zn²⁺ intercalation and Zn plating with mitigated HER in 1 M ZnCl₂ in H₂O:N-methyl-2-pyrrolidone (NMP) (1:17 mol/mol) electrolyte. Arrhenius analyses on the measured temperature-dependent nucleation overpotentials were conducted, revealing the critical role that ion adsorption plays prior to Zn plating or HER reactions. The presence of metallic Zn deposits was identified using optical microscopy and X-ray diffraction on galvanostatically discharged Mo₆Se₈ electrodes, confirming the occurrence of the Zn plating process on Mo₆Se₈.

It was verified that Zn can indeed plate on Mo₆Se₈ similar to how Li plating on graphite electrodes can occur in Li-ion batteries. This work enables the study of Zn plating as an analog to Li plating and establishes a connection between the two metal electroplating processes. Therefore, building a mechanistic understanding of both Zn and Li plating as failure modes could improve the practicality of each metal’s respective battery chemistries in all conditions.