Unraveling the Structure and Electrochemical Mechanism in TiNb₂O₇ via Operando Studies and Theoretical Calculations

Understanding the fundamental charge storage and conversion mechanism is of utmost important for developing and designing energy storage devices with exceptional electrochemical performances. In the quest for anodes, graphite and silicon are favored in the high-energy density application. However, they face challenges like overpotentials and lithium plating at high current densities. Recently, Nb-based oxides with a Wadsley–Roth crystallographic shear structure have been proposed as new anode materials for high-energy and high-power lithium-ion batteries (LIBs). The insertion of Li ions into Nb-based oxides mainly occurs at a voltage of about 1.6–1.7 V vs Li⁺/Li, preventing the electrolyte decomposition and lithium-dendrite formation. Among this class of materials, TiNb₂O₇ (TNO) is the most promising. Its theoretical capacity is 387.6 mAh g^–1 due to multielectron redox reactions involving several redox couples (Ti⁴⁺/Ti³⁺, Nb⁵⁺/Nb⁴⁺, Nb⁴⁺/Nb³⁺).
Unraveling the charge storage mechanism in the TNO is a challenging task because of complex crystal structure and the similar potentials for redox reactions of the transition metals. To investigate this a combined approach of experimental and theoretical analysis has been conducted.
Reversible charge–discharge capacity of the as-prepared TNO measured in a galvanostatic mode in the 1.0–3.0 V range at the 0.1C rate was 250 mAh g^–1. Cyclic Voltammetry during lithiation shows one large broad peak between 1.2V to 1.8V (vs Li⁺/Li) which corresponds to multiple reduction reactions of Nb⁵⁺ and Ti⁴⁺, because Ti 3d and Nb 4d states overlap in energy.
Operando Raman experiment reveals the order of redox reactions happening between Ti⁴⁺ and Nb⁵⁺. In the uncycled state, the Raman peaks at 998 cm^-1 and 884 cm^-1 are assigned to edge shared and corner shared NbO₆ octahedra. Similarly in the mid frequency region, the strong peaks at 647 cm^-1 and 538 cm^-1 are assigned to edge shared and corner shared TiO₆ octahedra. As soon as the discharge starts, the peak corresponds to corner shared NbO₆ disappears and the peak corresponding to edge shared NbO₆ undergoes red shift indicating the reduction of Nb⁵⁺/Nb⁴⁺ in the edge sharing octahedra site. At the same time both the peaks for TiO₆ undergo red shift confirming the reduction of Ti⁴⁺/Ti³⁺. At around 1.5V (vs Li⁺/Li), the peak position for edge sharing NbO₆ remain the unchanged, while the peak correspond to corner sharing NbO₆ keeps undergoing red shift revealing the further reduction of Nb^4+/3+ at the corner shared octahedra site. On the other hand, almost all the peaks corresponding to TiO₆ disappear starting at 1.7V and remain the same till the end of discharge. Quantum computational calculation has been performed to study electronic structure and to calculate the Raman modes with DFT using PBE functional.
To understand the Raman behavior and structural changes, in situ X-Ray diffraction has been performed. From Rietveld refinement, 5 distinct transitional metal sites (M1-M5) were identified. It has been observed that with the lithiation of TiNb₂O₇, the M5 site which has lowest Nb⁵⁺ occupancy and hence predominantly occupied by Ti⁴⁺, undergoes severe distortion. It can be comprehended that, due to this large distortion, the Raman peaks have disappeared once Ti⁴⁺ reduces to Ti³⁺. The refinement reveals the phase changes which supports our operando Raman analysis.