Decoding the Three-Card Monte: Unraveling the Role of Solvation Shell, Surface Adsorption, and SEI Formation on Zn Anode Performance

Additives or cosolvents are commonly used to curtail parasitic reactions in aqueous Zn-ion batteries. Usually, they are chosen based on the donor number, which indicates their affinity toward Zn2+. While their role in the modification of Zn-ion solvation shell, surface adsorption at the electrolyte/anode interface, and formation of solid–electrolyte interphase (SEI) are portrayed as a critical factors for enhancing Zn anode performance, deciphering the individual contributions is important to advance electrolyte engineering. In this work, we unveil the contrasting behaviors of two lactam cosolvents, caprolactam and 2-pyrrolidinone, in aqueous Zn-ion electrolytes. Although both electrolytes exhibit similar Zn-ion solvation structures and double-layer capacitances at the electrode/electrolyte interface, the caprolactam-based electrolyte outperforms its 2-pyrrolidinone counterpart. The Zn|Zn symmetric cell with a caprolactam-based electrolyte renders a cumulative capacity of ∼2600 mAh cm–2. Time-of-flight secondary-ion mass spectroscopy and in-situ FTIR measurements show the formation of a stable SEI through oligomerization of caprolactam. The importance of stable SEI formation as the key determinant in enhanced performance is further supported by crossover experiments. Overall, this study underscores the paramount importance of stable SEI formation over solvation and adsorption effects in enhancing the lifespan of Zn anodes.