Tracing Electrochemical Charge-Carrier Insertion into Vanadium Oxide Cathode in Aqueous Zinc-Ion Batteries

With growing concerns regarding the safety and cost issues of Li-ion batteries, extensive research is focusing on suitable alternatives. Among the candidates, aqueous electrolyte-based batteries have recently gained significant attention due to their non-flammable nature compared to organic electrolytes. As a water-based solution is utilized as the electrolyte, the role of protons, in addition to the primary redox charge carriers inserted in cathode materials, has become a crucial scientific issue in the field of aqueous battery research. Especially, under low-pH conditions, i.e., highly acidic electrolyte, the presence of protons becomes significant, potentially affecting the overall capacity and redox potential. Despite numerous studies investigating proton insertion into oxide cathodes, there is a lack of clear interpretations regarding the intercalation behavior. Therefore, to comprehensively understand the ambiguous role of protons in cathode materials within aqueous rechargeable batteries, it is essential to conduct systematic investigations covering a wide range of proton concentrations.<br/>Herein, we investigate the behavior of inserted charge carriers in V₂O₅ cathode oxide in two different cell types. In the presence of sufficient proton supply, such as beaker-cell configuration, the pH of the electrolyte influences the insertion process. Under low-pH conditions, we found that protons are inserted into the cathode before Zn ions. Otherwise, under high-pH conditions, Zn-ion serves as a major charge carrier, while the impact of protons on discharge voltage or capacity becomes less significant. To validate the generality of our hypothesis, we also demonstrate similar pH-dependent behavior in Na-, Mg- and Al-ion electrolytes. We established this mechanism through the integration of various analytical techniques. X-ray photoemission spectroscopy (XPS) spectra reveal valence state change of cathode materials and energy dispersive X-ray spectroscopy (EDS) elemental mapping directly visualizes and quantifies the Zn-ion and proton insertion. By employing a combination of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra, we gain insights into the overall oxidation state changes and bonding length alternations as a result of carrier-ion insertion. Furthermore, we examined the influence of electrolyte volume by adopting a coin-cell configuration. In this type of cell with a relatively small volume of acidic electrolyte, a substantial influence of protons observed in the low-pH beaker-cell test was not identified. Based on the aforementioned results, we conclude the proton contribution to discharge capacity is only significant when a sufficient amount of highly acidic electrolyte is utilized.