Correlation between cycling efficiency and surface morphology of electrodeposited lithium. Effect of fluorinated surface active additives

The cycleability of a lithium anode has been investigated in two solutions containing LiClO4: (i) a propylene carbonate (PC) solution and (ii) a blended solution consisting of PC, ethylene carbonate (EC) and 1,2-dimethoxyethane (DME) (volume RATIO = 1:1:2). Lithium and tetraethylammonium perfluorooctanesulfonate (LiFOS, TEAFOS) have been used as additives. Addition of these fluorinated surfactants at the optimum concentration, to electrolytic solutions, enhanced the lithium anode cycleability as the mean number of achieved cycles obtained is doubled in the presence LiFOS in PC and trebled (but with a larger dispersion) in the blended solution. The morphology of the lithium deposits has been observed by means of scanning electronic microscopy (SEM). SEM photographs of the lithium surface indicate that lithium deposited mainly as needle-like dendrites in PC---LiClO4 solutions and as a metallic sponge in the blended solvent. In the presence of the additives, the structure of electrochemically deposited lithium is modified and appears to be less porous due to an enhanced and more uniform nucleation. After a few cycles, modifications of the morphology of the lithium deposit are visible and correspond, in both electrolytic solutions, to a smoother surface with occasionally large cracks. The cycle efficiency appears to be related to the morphological properties of the lithium surface. The use of the surface-active additives LiFOS and TEAFOS that compete with the solvents to form more compact and uniform surface films on lithium, improves the cycling efficiency.