How Is Cycle Life of Three-Dimensional Zinc Metal Anodes with Carbon Fiber Backbones Affected by Depth of Discharge and Current Density in Zinc-Ion Batteries?

Zinc (Zn) metal is an attractive anode material for aqueous Zn-ion batteries (ZIBs). Three-dimensional (3D) carbon frameworks may serve as lightweight and robust hosts to enable porous Zn electrodes with a long cycle life. However, Zn electrode tests under a low depth of discharge (DOD) and current density often yield unreliable promises. We used 3D Zn electrodes with carbon nanofiber framework (CNF) backbones (Zn@CNF) as model electrodes to reveal how DOD and current density affect their performance. Plasma-treated CNFs provide sufficient surface hydrophilicity and surface area to allow uniform Zn plating/stripping of a thin and uniform Zn coating (5 mAh cm). CNFs only take a small weight fraction (17.5-19.7 wt. %) in the composite electrodes. The 3D structure and graphitic surface efficiently suppress dendrite growth. The cycle life of Zn@CNF can reach 843 h under 10% DOD and 0.5 mA cm in symmetric cells. However, high DOD and current density are detrimental to the stability of 3D Zn electrodes. The cycle life drops to 60.75 h under 60% DOD and 4 mA cm. Full cells assembled using Zn@CNF as anodes and VO as cathodes with an N/P capacity ratio of 2.4 delivered a capacity of 133.4 mAh g at 0.1 A g. The full cells also showed excellent capacity retention of 92.1% after 260 cycles under 0.5 A g with a high average DOD of 15.5%. Our results suggest that 3D Zn electrodes with CNF backbones are promising anodes for ZIBs. Studying Zn metal electrodes under practical DOD and current density is essential to access their potential accurately.