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

Abstract

Vanadium oxide (V 2 O 5 ) 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 V 2 O 5 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: LiClO 4 /propylene carbonate (PC) and LiPF 6 /diethyl carbonate (DEC) + ethylene carbonate (EC). Films cycled in LiPF 6 /DEC+EC experienced a 32% greater capacity fade between the first and second lithiathion/delithiation cycles than those cycled in LiClO 4 /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 V 2 O 5 cycled in LiPF 6 /DEC+EC contained a high atomic concentration percentage of fluoride (16.18%) in comparison with V 2 O 5 electrodes cycled in LiClO 4 /PC (3.94%). No significant amounts of carbonates, oxalates, or oxyfluorophosphates typically associated with SEI formation were found when V 2 O 5 was cycled in either electrolyte. The results obtained suggest instead that HF, formed upon water contamination of the electrolyte, reacts with V 2 O 5 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.