Potential-Dependent ATR-SEIRAS and EQCM-D Analysis of Interphase Formation in Zinc Battery Electrolytes

With the heightening interest in bivalent battery technology, there arises a necessity for a thorough investigation into zinc-ion battery (ZIB) electrolytes, accommodating their chemical attributes and potential-dependent structural dynamics. While the phenomenon of in situ solid electrolyte interphase formation is extensively documented in lithium-ion batteries, its analogous occurrences in ZIBs remain limited. Herein is a comparative study of three zinc electrolytes of interest: ZnSO₄, ZnOTF, and Zn(TFSI)₂/LiTFSI hybrid water-in-salt electrolyte. Additionally, the impact of an acetonitrile additive is scrutinized, with a comparative assessment of the interfacial behavior in aqueous solutions. Utilizing ATR-SEIRAS, potential-dependent alterations in the composition of the electrolyte/electrode interface were monitored, while EQCM-D facilitated a comprehensive understanding of variations in the mass and structural properties of the adsorbed layer. Aqueous ZnSO₄ demonstrated the accumulation of porous Zn₄SO₄(OH)₆·<i>x</i>H₂O at negative potentials, leading to a mass of 1.47 μg cm^-2 after five cycles. Bisulfate formation was observed at positive potentials. SEIRAS measurements for ZnOTF demonstrated reorientation and surface adsorption of CF₃SO₃^- to favor CF₃ at the surface for positive potentials, and acetonitrile showed increased stability for the electrode at negative potentials. The additive was also reported to lead to the accumulation of a substantial passivation layer with viscoelastic properties. The zinc water-in-salt showed exceptional surface stability at negative potentials and a widened potential window. A thin rigid zinc SEI layer is reported with a mass of 0.7 μg cm^-2. The compositional intricacies of these surface structures are discussed in relation to their solvent conditions. This investigation not only sheds light on the initial charge/discharge cycles in ZIBs but also underscores their pivotal role in instigating enduring transformations that can significantly influence their long-term cycling performance.