Electrodeposited tin coating as negative electrode material for lithium-ion battery in room temperature molten salt

Abstract

A new room temperature molten salt (RTMS) [1-methyl-3-ethylimidazolium/AlCl3/SnCl2 (3:2:0.5)] was developed for depositing tin on a copper electrode. Different tin crystallites were deposited at different temperatures, giving widely different performances of the assembled lithium cell [Sn (Cu)/LiCl buffered MEICl-AlCl3 RTMS/lithium]. Tin film deposited at 50°C or higher gave a more desirable crystal structure and an improved performance than films obtained at lower temperatures. Both cyclic voltammetry and galvanostatic cycling show the formation of three major lithium-tin alloy phases corresponding to the phase transition of LiSn/Li7Sn3, Li13Sn5/Li7Sn2, and Li7Sn2/Li22Sn5. Increases in the charging and discharging capacities were found with the deposition of higher lithium-rich tin alloys, though at the degradation of the irreversible capacity at the first cycle. The discharging capacity decreased rapidly, producing loose, expanded, and irregular crystallites upon cycling at a high current density (cd) (1.0 mA/cm2). However, an average capacity of 140 mAh/g, coulombic efficiency around 85%, and more than 200 cycles were obtained at a low cd (0.4 mA/cm2). The improvement is attributed to the deposition of small and regular tin crystallites that allows reversible insertion and removal of lithium from a more stable crystal structure without a significant volume change during cycling.