Oxide Acidity Modulates Structural Transformations in Hydrogen Titanates During Electrochemical Ion Insertion

Diversifying the materials chemistry landscape is important for alleviating supply chain issues for next generation electrochemical energy storage. Titanium oxides are attractive as electrodes for energy storage applications due to the terrestrial abundance and low toxicity of Ti. Hydrogen titanates (HTOs) form a diverse group of metastable, layered titanium oxides with an interlayer containing both water molecules and structural protons. We investigated how the chemistry of this interlayer environment influences electrochemical ion insertion from aqueous and non-aqueous electrolytes and correlated the electrochemical response with the physical and chemical properties of HTOs using a suite of characterization tools, including operando and in situ methods. In aqueous acidic electrolytes, we found that the nature of the acid (inorganic vs. organic) dictates the selectivity of the proton-coupled electrochemical reactions and that the primary degradation mechanism is via Ti³⁺ dissolution. In the case of non-aqueous aprotic electrolytes, we found that structural protons were necessary for activating HTOs towards Li⁺ insertion, and that the potential of the first reduction reaction trended with the relative acidity of the structural protons. We propose that the initial electrochemical reaction in the non-aqueous electrolyte involves reduction of the structural protons to yield hydrogen gas and formation of a lithium titanate. The hydrogen gas is initially confined within the HTO lattice until the titanate structure expands upon oxidation. Our work has broad implications for the electrochemical behavior of insertion materials containing hydrogen and structural water molecules, where hydrogen evolution is expected at potentials below the hydrogen reduction potential and in the absence of water or protons. This behavior is distinct to the aqueous electrochemistry of these materials, and provides an example of electrochemical electron transfer to a non-metallic element in a solid-state host, in analogy to anion redox.