Influence of Hydrofluoric Acid Formation on Lithium Ion Insertion in Nanostructured V₂O₅

Vanadium oxide (V₂O₅) is a multifaceted material possessing desirable redox properties, including accessibility to multiple valence states, which make it attractive as a cathode for lithium ion batteries and microbatteries. Studies show that performance of this electrode material is dependent on the electrolyte employed and that solid electrolyte interphase (SEI) layer formation is responsible for the fade in capacity with multiple cycling. Nanostructured V₂O₅ thin films synthesized through reactive ballistic deposition (RBD) were studied with electrochemical methods, ex situ Raman and ex situ XPS in two widely used electrolytes: LiClO₄/propylene carbonate (PC) and LiPF₆/diethyl carbonate (DEC) + ethylene carbonate (EC). Films cycled in LiPF₆/DEC+EC experienced a 32% greater capacity fade between the first and second lithiathion/delithiation cycles than those cycled in LiClO₄/PC, due to a redox-induced change in the surface morphology and composition and an irreversible transformation into an amorphous state as monitored by ex situ Raman. From X-ray photoelectron spectroscopy (XPS), it was shown that V₂O₅ cycled in LiPF₆/DEC+EC contained a high atomic concentration percentage of fluoride (16.18%) in comparison with V₂O₅ electrodes cycled in LiClO₄/PC (3.94%). No significant amounts of carbonates, oxalates, or oxyfluorophosphates typically associated with SEI formation were found when V₂O₅ was cycled in either electrolyte. The results obtained suggest instead that HF, formed upon water contamination of the electrolyte, reacts with V₂O₅ through a self-catalyzed process both at open circuit and under applied potential. The formation of vanadium oxyfluorides causes active mass loss and severe capacity fade upon discharging/charging