Distinguish MnO2/Mn2+ Conversion/ Zn2+ Intercalation/ H+ Conversion Chemistries at Different Potentials in Aqueous Zn||MnO2 Batteries

Abstract

The rechargeable aqueous Zn||MnO<inf>2</inf> chemistry has been extensively explored, but its electrochemical reaction mechanisms, especially in the context of MnO<inf>2</inf>/Mn²⁺ conversion and Zn²⁺/H⁺ intercalation chemistry, remain not fully understood. Here, we designed an amphiphilic hydrogel electrolyte, which distinguished the MnO<inf>2</inf>/Mn²⁺ conversion, Zn²⁺ intercalation, and H⁺ intercalation and conversion processes at three distinct discharge plateaus of an aqueous Zn||MnO<inf>2</inf> battery. The amphiphilic hydrogel electrolyte is featured with an extended electrochemical stability window up to 3.0 V, high ionic conductivity, Zn²⁺-selective ion tunnels, and hydrophobic associations with cathode materials. This specifically designed electrolyte allows the MnO<inf>2</inf>/Mn²⁺ conversion reaction at a discharge plateau of 1.75 V. More interesting, the discharge plateaus of ~1.33 V, previously assigned as the co-intercalation of Zn²⁺ and H⁺ ions in the MnO<inf>2</inf> cathode, are specified as the exclusive intercalation of Zn²⁺ ions, leading to an ultra-flat voltage plateau. Furthermore, with a distinct three-step electrochemical energy storage process, a high areal capacity of 1.8 mAh cm⁻² and high specific energy of 0.858 Wh cm⁻², even at a low MnO<inf>2</inf> loading mass of 0.5 mg cm⁻² are achieved. To our knowledge, this is the first report to fully distinguish different mechanisms at different potentials in aqueous Zn||MnO<inf>2</inf> batteries.