Comprehensive Study on the Role of Proton in the Insertion Chemistry of Aqueous Zn-Ion Batteries
Authors: Lee Changmin , Kim Dongho , Hong Youngjae , Lim Younghwan , Park Chang Hyun , Chung Sung-Yoon 
Published: December 2024

Li-ion rechargeable batteries played a major role in the present revolution of portable devices since Goodenough's group switched from sulfide to oxide cathode materials in the 1980s. However, the safety of explosive organic electrolytes, rising demand-driven costs, and even geopolitical concerns over lithium minerals have prompted scientists to seek alternate systems. Due to their enormous capacity, Zn-ion rechargeable batteries with a non-explosive aqueous electrolyte have garnered significant attention among the candidates. Especially the newly demonstrated capacity over 300 mAh/g in the V2O5 cathode illustrated the promise of the aqueous Zn-ion system. Although aqueous electrolytes reduce safety concerns, how protons function in insertion chemistry is still unclear. In contrast to Li-ion's single insertion behavior in organic electrolytes, charge carriers compete with one another, particularly in low-pH regions with a lot of proton content. Therefore, a thorough examination of insertion behavior over a broad pH range is necessary to comprehend the involvement of protons.

In this work, we systematically investigated the insertion behavior in the V2O5 cathode in two distinct cell types in three pH conditions. To guarantee reliable results, we integrated multi-scale analytical methods and selected compact sampling points from discharge curves. Our first cell type, the beaker cell, provided sufficient protons comparable to metal cations. The insertion behavior in this environment was dependent on the proton concentration. EDS Elemental maps from samples with greater pH levels reveal Zn signals are found even in the early stages of discharge, despite the fact that protons are inserted first in low-pH samples (Fig. 1a). This implies that Zn-ion plays a major role as a charge carrier while influence of protons on discharge voltage or capacity becomes less significant under mildly acidic conditions. Additionally, the vanadium’s valence state changed as the discharge proceed, more reduced when Zn-ion is inserted instead of proton in the high-pH region, according to the XPS spectrum from identical samples with EDS maps. We employed X-ray absorption spectroscopy techniques to overcome the shortcomings of the surface sensitivity of XPS and local region observation in STEM. The XAS results provided insights about the overall oxidation state changes of vanadium as a result of carrier-ion insertion, where the XANES spectrum shows a modest reduction with proton insertion and a notable reduction upon the Zn-ion insertion. The bonding nature was further explained by the simultaneously obtained EXAFS spectrum, which showed the characteristic peak of the Zn insertion site and the peak splitting of V-V bonding under high pH circumstances. In addition to Zn-ion electrolytes, we also revealed similar pH-dependent insertion behavior in monovalent Na-, divalent Mg-, and trivalent Al-ion electrolytes to generalize our hypothesis.

Coin cell cases show the opposite of the beaker cell test, where the proton has a negligible effect on the insertion chemistry. Regardless of proton concentration, we discovered that Zn-ions are always inserted from the first step of discharge, as evidenced by elemental maps in Fig. 1b. The oxidation state variation of vanadium in the XPS spectrum and even redox peak position in CV curves for every pH was identical, which can be somewhat inferred from similar discharge curve shapes irrespective of pH. We concluded that the proton contribution to discharge capacity is only meaningful when a substantial amount of highly acidic electrolyte is supplied. Thus, it is essential to consider the relative mass ratio between the electrolyte volume and cathode material mass to ascertain how protons affect the voltage and capacity of an aqueous battery system.

Figure 1