Extreme Cycling Longevity in Additive Free T-Nb2O5 over a Quarter Million Lithiation Cycles

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Rapid energy storage via intercalation requires quick ionic diffusion and often results in pseudocapacitive behavior. The long-term cycling stability of such energy storage materials remains understudied despite the relevance to their lifetime cost and market feasibility. Orthorhombic niobium oxide (T-Nb2O5)is a rapid ion intercalation material with a theoretical lithiation capacity of 201.7 mAh g-1 (Li2Nb2O5) and good cycling stability due to the minimal unit cell strain during (de)intercalation. Prior reports of T-Nb2O5 cycling between 1.3-3.1 V vs. Li/Li+ noted a 50% loss in capacity after 10k cycles, stemming from a convolved effect of amorphization, dissolution, and/or delamination. Here, cyclic voltammetry is used to identify the role of the voltage window, state of charge (SOC), and potentiostatic holds on the cycling stability of mesoporous T-Nb2O5 thin films. Films cycled at high-rate between 1.2-3.0 V vs. Li/Li+ without voltage holds (kinetically limited SOC of Li1.1Nb2O5) exhibited extreme cycling stability with 90.8% ± 2.1% capacity retention after a quarter million cycles without detectable morphological/crystallographic changes.1 In contrast, inclusion of 60 s voltage holds (SOC of Li2.18Nb2O5) led to rapid capacity loss with only a 61.6% retention after 10k cycles and corresponding x-ray diffraction evidence of amorphization. Cycling with other limited voltage windows identified that most crystallographic degradation occurred at higher extents of lithiation (<1.4 V vs Li/Li+). These results reveal remarkable stability, up to 25x higher than what has been previously observed, over limited conditions and suggest that T-Nb2O5 amorphization is associated with high extents of lithiation. [1] Wechsler, S. C.; Gregg, A.; Stefik, M. Highly Reversible Lithiation of Additive Free T-Nb2O5 for a Quarter of a Million Cycles. Adv. Funct. Mater. 2023, ASAP. 10.1002/adfm.202312839.