Separator-free Zn-ion Battery with Mn:V3O7·H2O Nanobelts and a Zn2+-Polyacrylamide Semisolid Electrolyte with Ultralong Cycle Life.

Abstract

Vanadium based oxides are immensely suitable for zinc-ion-batteries (ZIBs) due to their layered and stable crystal structures. In this study, Mn doped V3O7·H2O nanobelts were synthesized and used as cathodes in ZIBs for the very first time and the doped oxide exhibited an enhanced capacity of 258 mAh g-1 compared to its undoped counterpart (208 mAh g-1) at the same current density of 40 mA g-1. Mn:V3O7·H2O outperforms the V3O7·H2O due to the superior bulk electrical conductivity as well as higher nanoscale current carrying capability imparted by a high proportion of mixed valent states of Mn3+, Mn2+, V5+, and V4+ and the smaller crystallite size that affords short diffusion lengths for Zn2+ ions. The Mn:V3O7·H2O cathode is coupled with a Zn2+ ion conducting polyacrylamide gel electrolyte and a Zn flakes/activated carbon (Zn Fs/C) composite anode to yield a unique separator free Mn:V3O7·H2O/Zn2+-PAM gel/Zn-Fs/C battery. The cell exhibits a capacity of ∼205 mAh g-1 (at 40 mA g-1) and retains 99% of its original capacity after 3500 cycles. The Zn2+-PAM gel shows a high ionic conductivity in the range of 5.9 to 28.2 S cm-1, over a wide temperature span of 0 to 70 °C, and a wide electrochemical potential stability window of -0.5 to +2.3 V, thus rendering it suitable for low temperature applications as well. The gel also inhibits dendritic growth of Zn over the Zn-Fs/C anode through regulated flow of Zn2+ ions during charging, prevents cathode dissolution, and improves cycle life via preservation of structural integrity of the Mn:V3O7·H2O cathode after 200 charge-discharge cycles. This is a highly scalable cell configuration and opens up opportunities to produce long lasting batteries completely free of costly separators with a semisolid free-standing electrolyte and a robust doped oxide.