Slow Scan Cyclic Voltammetry of Li-Ion Insertion in T‑Nb₂O₅ Reveals Hidden Peaks and Multi-Electron Redox

The orthorhombic phase of Nb₂O₅ formed at low temperatures (T-Nb₂O₅) is a promising alternative anode material for lithium-ion batteries due to its ability to reversibly (de)-lithiate at high rates without forming lithium metal, which is a major safety limitation of conventional graphite anodes. Despite decades of research, the cyclic voltammetry response of T-Nb₂O₅ remains poorly understood, with conflicting reports regarding the number and peak potentials of cathodic and anodic redox peaks in the voltammogram. While some studies report a single broad redox feature in cyclic voltammograms, others observe multiple peaks, yet often describe lithiation using a single overall reaction: T-Nb₂O₅ + xLi⁺ + xe ^-↔T-Li _x Nb₂O₅ for 0 < x < 2. For x = 2, Li insertion is accompanied by single-electron reduction of each Nb center (Nb⁵⁺ → Nb⁴⁺). In this work, we employ slow-scan cyclic voltammetry (SSCV) at an ultraslow rate of 1.5 μV/s to minimize resistive losses and kinetic limitations in the oxide. Under these near-equilibrium conditions, we achieve x = 3.0 in T-Li _x Nb₂O₅ from 3.0-1.2 V, corresponding to more than one electron per Nb center, and resolve five distinct cathodic peaks during lithiation. Three broad peaks are assigned to structural transformations based on recent literature in situ synchrotron-based X-ray diffraction data [HanH., . Nat. Mater.2023, 22-(9), 1128-1135.]: an orthorhombic-to-monoclinic distortion at x ≈ 0.5, followed by transitions to an amorphous insulating phase, and a tetragonal Li-rich layered rock salt structure at x = 2.0 and x = 2.5, respectively. In addition, two sharp peaks at 1.678 and 1.658 V, which merge into a single "super peak" at faster sweep rates (>100 μV/s), are attributed to surface-related electrochemical processes such as Nb reduction and Li-ion adsorption, surface phase transitions, or impurity B-Nb₂O₅. These features, widely observed in the literature but previously unassigned, underscore how electrode architecture (particles vs thin films), morphology, and scan rate conditions shape the voltammetric signature of T-Nb₂O₅. By employing ultraslow scan rates to suppress kinetic limitations and ohmic losses, we reveal previously hidden cathodic features in the cyclic voltammograms and directly correlate them with recently reported in situ structural phase transitions.