Investigating the Degradation of Nb₂O₅ Thin Films Across 10,000 Lithiation/Delithiation Cycles

Nb2O5 is a Li+ intercalation transition metal oxide that is of current interest for lithium ion battery and capacitor electrodes. For orthorhombic (T) Nb2O5 films prepared by electrophoretic deposition (EPD) and subjected to lithiation/delithiation cycling, a remarkably reproducible degradation process is observed. It is characterized by the onset of irreversible capacity loss from a baseline specific capacity, Csp, of 400 (±50) F/g at 1000 (± 500) cycles. A gradual reduction of Csp occurs during the ensuing 9000 cycles after which the Csp stabilizes at 200 (±25) F/g. We investigate this degradation using six ex situ instrumental methods and more than 100 individual Nb2O5 films to characterize and understand the composition, atomic scale structure, chemical bonding, electrochemical, and electrical properties of these films during these 10,000 cycles. What emerges is a multidimensional picture of the degradation process in which the decline in Csp occurs concurrently with an increase in the charge transfer resistance, a loss of crystalline order, and the dissolution of niobium from the film.