Salt-concentrated acetate electrolytes for a high voltage aqueous Zn/MnO2 battery

Aqueous rechargeable Zn/MnO2 batteries are attractive due to their low-cost, high safety and use of non-toxic materials. In term of electrolyte materials, it is anticipated that an aqueous electrolyte with a wider electrochemical window will improve the stability and energy density. In this work, we investigated salt-concentrated electrolytes based on relatively inexpensive acetate salts. An electrochemical window of 3.4 ​V was achieved in salt-concentrated 1 ​m Zn(OAc)2+31 ​m KOAc electrolyte. Its total ionic conductivity is 2.96 ​× ​10-2 ​S ​cm-1 while the ionic conductivity of Zn2+ ions is 7.80 ​× ​10-3 ​S ​cm-1, estimated by a current interrupt method. This electrolyte is regarded as a mild alkaline environment with a pH value of 9.76, causing the different storage mechanism for anode with Zn2+ ions and, cathode with OH- ions as the charge carriers respectively. A Zn/MnO2 battery was assembled using 1 ​m Zn(OAc)2+31 ​m KOAc electrolyte, self-supported α-MnO2-TiN/TiO2 cathode and Zn foil anode. The Zn/MnO2 battery can be charged to 2.0 ​V versus Zn/Zn2+ and delivers discharge capacity and energy density of 304.6 ​mAh·g-1 (calculated on the mass of MnO2) or 0.32 mAh·cm-2 (calculated on the area of electrode) and, 368.5 ​Wh·kg-1 (calculated on the mass of MnO2) or 232.7 Wh·kg-1 (calculated on the total active mass of electrodes and electrolyte) in the first cycle under a current density of 100 mA·g-1 (~ C/3, based on the mass of MnO2) or 0.1 mA·cm-2 (based on the area of electrode). During cycling, the coulombic efficiency can be maintained around 99% and reached 99.9% during the 14-340th cycles. After the cycling tests, almost no dendrites were observed on the Zn foil anode attributing to the super-high salt concentration in that acetate-based electrolyte, which will benefit the stability of aqueous Zn/MnO2 batteries.