Ionic liquid gating (ILG) technique has been demonstrated as a powerful manner to manipulate the valence-state and electronic structures of complex oxides through topotactic phase transformations. However, the dominant type of transfer ions of a particular system and the underlying mechanisms for the ion incorporation dynamics remain controversies in different systems. In this article, ILG treatments were applied to double-perovskite ${\mathrm{La}}_2{\mathrm{NiMnO}}_6$ (LNMO) system. In response to positively gated electric bias, protons (${\mathrm{H}}^{+}$) incorporate into LNMO films gradually. Based on the combined results from synchrotron-based x-ray absorption and photoelectron spectroscopies, the doped electrons not only induce the valence-state reductions of both Mn and Ni sites, but also the downshift of the valence-band maximum away from the Fermi level, which is distinct from the valence-state and electronic-state variations induced by electron doping from oxygen vacancies (${\mathrm{V}}_{\mathrm{o}}$). This peculiar valence-state reduction has been verified by first-principles calculations, which not only enlightens the roles of energy competitions between ${\mathrm{H}}^{+}$ insertion and ${\mathrm{V}}_{\mathrm{o}}$ formation, but also the electronic structures near the Fermi level in determining the electrochemical gating mechanism of complex oxides.

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• Electronic structure
• Thin films
• Density functional theory
• X-ray absorption spectroscopy