Cathode-Electrolyte Interphase in a LiTFSI/Tetraglyme Electrolyte Promoting the Cyclability of VO.

VO, one of the earliest intercalation-type cathode materials investigated as a Li host, is characterized by an extremely high theoretical capacity (441 mAh g). However, the fast capacity fading upon cycling in conventional carbonate-based electrolytes is an unresolved issue. Herein, we show that using a LiTFSI/tetraglyme (1:1 in mole ratio) electrolyte yields a highly enhanced cycling ability of VO (from 20% capacity retention to 80% after 100 cycles at 50 mA g within 1.5-4.0 V vs Li/Li). The improved performance mostly originates from the VO electrode itself, since refreshing the electrolyte and the lithium electrode of the cycled cells does not help in restoring the VO electrode capacity. Electrochemical impedance spectroscopy (EIS), post-mortem scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the origin of the improved electrochemical behavior. The results demonstrate that the enhanced cyclability is a consequence of a thinner but more stable cathode-electrolyte interphase (CEI) layer formed in LiTFSI/tetraglyme with respect to the one occurring in 1 M LiPF in EC/DMC (1:1 in weight ratio, LP30). These results show that the cyclability of VO can be effectively improved by simple electrolyte engineering. At the same time, the uncovered mechanism further reveals the vital role of the CEI on the cyclability of VO, which can be helpful for the performance optimization of vanadium-oxide-based batteries.