Characterization of Li Diffusion and Solid Electrolyte Interface for Li4Ti5O12 Electrode Cycled with an Organosilicon Additive Electrolyte

Abstract

The use of lithium titanate (Li4Ti5O12, LTO) for the negative electrode in lithium ion batteries has attracted enormous attention owing to its fast charging capability, high power, safe operating voltage window and stable structure (“zero strain”) during cycling. Researchers have investigated the formation of the solid electrolyte interface (SEI) of the LTO electrode, which prevents gassing issue and leads to longer cycle life. In this study, the solid-state diffusion property of LTO at room temperature was characterized using AC impedance spectroscopy at different states of charge (SOC) during charge and discharge to reveal the dependency of the lithium diffusion coefficient on SOC. Meanwhile the formation and growth of the solid electrolyte interface (SEI) on the LTO electrode using an electrolyte containing Silatronix OS3® additive were investigated using X-ray photoelectron spectroscopy (XPS). The composition of the SEI and its evolution due to cycling with the OS3® additive was compared to that with a commercial electrolyte. Half-cell coin cells of LTO vs lithium metal were formed and cycled at room temperature for over 200 cycles, where the resistance increase, as measured by impedance spectroscopy, is correlated to the SEI growth. Electrode samples were analyzed in the pristine state, after formation, and after 200 cycles. XPS results showed that a thin layer of SEI is formed during the first two formation cycles and the composition of the SEI on the surface of the LTO electrode varied with increasing cycle number. Based on the escape depths of Ti 3 s and Ti 2p regions, the SEI after formation is thicker than 5.5 nm but is less than 7.0 nm for both the OS3® and A7 electrolytes. Based on Ar-ion depth profiling, the SEI thickness in terms of the equivalent thickness of SiO2 after 200 cycles in coin cell configuration is estimated to be near 14 nm for both the OS3® and A7 electrolytes. A much higher fluorine content (F 1s peak) was found in the SEI formed with the OS3® electrolyte than the SEI formed with the commercial A7 electrolyte.