Electrodeposited Tin Coating as Negative Electrode Material for Lithium-Ion Battery in Room Temperature Molten Salt

A new room temperature molten salt (RTMS) [ 1 ­ methyl ­ 3 ­ ethylimidazolium / AlCl 3 / SnCl 2 (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 ­ AlCl 3 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 / Li 7 Sn 3 , Li 13 Sn 5 / Li 7 Sn 2 , and Li 7 Sn 2 / Li 22 Sn 5 . 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 / cm 2 ) . 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 / cm 2 ) . 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.