An unconventional charge compensation mechanism for proton insertion in aqueous Zn-ion batteries†

Aqueous Zn-ion batteries have been proposed as safe and economical options for large-scale energy storage. In theory, they operate by reversibly shuttling zinc ions between a metallic zinc anode and a cathode material for Zn²⁺ ion intercalation through an aqueous electrolyte of a zinc salt solution. In practice, protons (H⁺) in the aqueous electrolyte can compete with and even predominate Zn²⁺ in the intercalation reaction. A diagnostic consequence of H⁺, as opposed to Zn²⁺, insertion is the precipitation of layered double hydroxide (LDH) crystals, which can be readily identified by electron microscopy and X-ray diffraction measurements. Absence of LDH formation has been perceived as evidence for Zn²⁺ insertion. Using a combination of X-ray diffraction, electron microscopy, and X-ray photoelectron spectroscopy, we reveal a different charge compensation mechanism in a vanadyl phosphate electrode, where H⁺ insertion predominates in an aqueous Zn(CF₃SO₃)₂ electrolyte. The H⁺ insertion induces a conformal deposition of an amorphous ZnO layer on the electrode particle, which cannot be captured by scanning electron microscopy or X-ray diffraction. Our work underlines the complexity of the charge compensation mechanism in aqueous Zn-ion batteries, which is relevant to other multivalent systems.