Cathodic Zn underpotential deposition: an evitable degradation mechanism in aqueous zinc-ion batteries.

During cycling, Zn is found to be deposited on the cathode at a potential well above its standard deposition potential. Zn x (OTf) y (OH) 2 x−y · n H 2 O derived from H+ insertion is the deposition matrix. This underpotential deposition follows a 2D nucleation and growth model and is proposed to be thermodynamically feasible. By controlling the end-of-discharge voltage, cathodic Zn deposition is removed and battery's degradation is significantly mitigated. [Display omitted] Aqueous zinc-ion batteries (AZIBs) are promising for large-scale energy storage, but their development is plagued by inadequate cycle life. Here, for the first time, we reveal an unusual phenomenon of cathodic underpotential deposition (UPD) of Zn, which is highly irreversible and considered the origin of the inferior cycling stability of AZIBs. Combining experimental and theoretical simulation approaches, we propose that the UPD process agrees with a two-dimensional nucleation and growth model, following a thermodynamically feasible mechanism. Furthermore, the universality of Zn UPD is identified in systems, including VO 2 //Zn, TiO 2 //Zn, and SnO 2 //Zn. In practice, we propose and successfully implement removing cathodic Zn UPD and substantially mitigate the degradation of the battery by controlling the end-of-discharge voltage. This work provides new insights into AZIBs degradation and brings the cathodic UPD behavior of rechargeable batteries into the limelight. [ABSTRACT FROM AUTHOR]