Slow scan cyclic voltammetry of Li-ion insertion in T-Nb₂O₅ reveals hidden peaks and multi-electron redox

Abstract

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"-" Nb〗_2 O_5+〖xLi〗^++〖xe〗^-↔〖"T-Li" 〗_"x" 〖"Nb" 〗_"2" "O" _"5" for 0 < x < 2. In this work, we employ slow-scan cyclic voltammetry (SSCV) at an ultra-slow 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-LixNb2O5 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 (Han, H.; et al. 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 solid electrolyte interphase formation. 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. SSCV reveals previously unresolved redox features, providing new insight into the stepwise lithiation behavior of T-Nb2O5 and refining our understanding of its electrochemical mechanism.