Bulk Proton Acidity Modulates Structural Transformations in Hydrogen Titanates during Li⁺-Insertion Coupled Electron Transfer

Nanostructured TiO₂-based materials are of interest for energy storage and conversion applications due to their high abundance, low toxicity, and chemical tunability. Lithium titanium oxides, such as spinel-type Li₄Ti₅O₁₂ and ramsdellite-type Li₂Ti₃O₇, have been extensively studied as electrode materials for Li-ion batteries. The synthesis of nanostructured metal oxide materials with high surface area has been a promising path for achieving high specific capacity for electrochemical energy storage. As such, it is important to expand the materials design strategies to synthesize titanium oxide materials for high Li-ion energy storage applications. Here we explore a series of layered hydrogen titanates (HTOs), H₂Ti_nO_2n+1●xH₂O (n = 3, 4, and 5), with varying degrees of structural protonation and interlayer water. These HTOs are metastable materials prepared via acid etching of alkali titanates. The complexity of the interlayer environment motivated us to understand the role of water content and structural protons on Li⁺-insertion coupled electron transfer from a non-aqueous electrolyte. We hypothesized that structural protons are necessary for high Li⁺ intercalation capacity, while the mobility of these protons determines the degree of structural change that occurs during Li⁺ insertion. To investigate these phenomena, we utilized electrochemistry as well as operando electrochemical XRD, ex situ solid-state NMR, and acid-base titrations. By employing these techniques, we correlated the relative acid strengths of the structural protons to the degree of structural change during Li⁺ intercalation. Our findings show that HTOs with more acidic protons undergo rapid, irreversible structural changes due to hydrogen evolution of bulk protons. Long term electrochemical cycling reveals irreversible structural transformations of the HTOs to lithiated titanates. This work provides a comprehensive understanding of the relationship between bulk proton acidity and structural transformations during electrochemical Li⁺ intercalation into titanates. In doing so, it informs the structural design of next generation ion-insertion coupled electron transfer materials for energy storage and conversion.