Effects of Some Organic Additives on Lithium Deposition in Propylene Carbonate

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Abstract
The effects of some film-forming organic additives, fluoroethylene carbonate (FEC), vinylene carbonate (VC), and ethylene sulfite (ES), on lithium deposition and dissolution were investigated in 1 M LiClO4LiClO4 dissolved in propylene carbonate (PC) as a base solution. When 5 wt % FEC was added, the cycling efficiency was improved. On the contrary, addition of 5 wt % VC or ES significantly lowered the cycling efficiency. The surface morphology of lithium deposited in each electrolyte solution was observed by in situ atomic force microscopy (AFM). In PC+FEC,PC+FEC, the surface was covered with a uniform and closely packed layer of particle-like deposits of about 100-150 nm diam. The surface film seemed to be more solid in PC+VC,PC+VC, and inhomogeneous in PC+ES.PC+ES. From ac impedance measurements, it was revealed that the surface film formed in PC+FECPC+FEC has a lower resistance than that in the additive-free solution, whereas that formed in PC+VCPC+VC or PC+ESPC+ES has a higher resistance. Large volume changes during lithium deposition and dissolution require that the surface film should be elastic (or soft) and be self-repairable when being damaged. In addition, a nonuniform current distribution is liable to cause dendrite formation, which requires that the surface film should be uniform and its resistance should be as low as possible. PC+FECPC+FEC gave a surface film that satisfies all these requirements, and therefore only FEC was effective as an additive for deposition and dissolution of lithium metal. © 2002 The Electrochemical Society. All rights reserved.