Electrolyte pH and operating potential: critical factors regulating the anodic oxidation and zinc storage mechanisms in aqueous zinc ion battery

Abstract

The complicated multi-electron reactions and electrochemical/chemical evolution of vanadium oxides in zinc ion batteries make the detailed reaction mechanism difficult to understand. Here, we show that these reactions can be easily interpreted from the reliable potential–V-species (electrochemical) and pH–V-species (chemical) relationships. A systematic investigation of several low-valence vanadium oxides (V6O13, VO2, and V2O3) is conducted to study the detailed reaction mechanisms upon the anodic oxidation and routine zinc storage process. Vanadium oxides are thermodynamically converted to layered zinc pyrovanadate Zn3V2O7(OH)2·2H2O (ZVOH) in a certain range of pH (4.0–8.5) and operating potential following the VOx oxidation →V-species dissolution →ZVOH precipitation processes. The establishment of pH–V-species and potential–V-species relationships can well explain the reaction mechanisms not only in anodic oxidation but also in routine zinc insertion/extraction processes with the combination of electrochemical and chemical reactions. Moreover, due to the favorable open and stable crystal framework and unique Zn2+ storage mechanism, the obtained ZVOH exhibits a highly improved capacity of 492.7 mAh/g within the voltage window of 0.3–1.4 V. Our work highlights the critical roles of operating potential and electrolyte pH in zinc ion battery and offers new knowledge and practices for its commercialization.