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

Vanadium based oxides are immensely suitable for zinc-ion-batteries (ZIBs) due to their layered and stable crystal structures. In this study, Mn doped V 3 O 7 ·H 2 O 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:V 3 O 7 ·H 2 O outperforms the V 3 O 7 ·H 2 O 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 Mn 3+, Mn 2+, V 5+, and V 4+ and the smaller crystallite size that affords short diffusion lengths for Zn 2+ ions. The Mn:V 3 O 7 ·H 2 O cathode is coupled with a Zn 2+ ion conducting polyacrylamide gel electrolyte and a Zn flakes/activated carbon (Zn Fs/C) composite anode to yield a unique separator free Mn:V 3 O 7 ·H 2 O/Zn 2+ -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 Zn 2+ -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 Zn 2+ ions during charging, prevents cathode dissolution, and improves cycle life via preservation of structural integrity of the Mn:V 3 O 7 ·H 2 O 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.