Abstract
A series of lithium difluoro-2-fluoro-2-alkyl-malonatoborate salts have been used as additives in conventional 1.0 M LiPF6/ethylene carbonate (EC)-dimethyl carbonate (DMC)-diethyl carbonate (DEC) (1-1-1, by v) electrolyte for high voltage LiNi0.5Mn1.5O4 (LNMO) batteries. Cyclic voltammograms (CVs) reveals that the electrolytes with additives can significantly suppress the co-intercalation of solvents into the graphene layers during the first cycle, simply because of their sacrificial reductions on the surface of the graphite electrode above 1.0 V vs Li/Li+. In addition, electrochemical floating test shows less oxidation current in the electrolytes with additives at voltages above 5.0 V, proving good oxidation protection by the additives. More importantly, the presence of additives can effectively increase the first cycle coulombic efficiencies and long cycling stability in LNMO based half-cells and full cells. SEM and XPS survey of the cycled anode and cathode electrodes provide evidence for the formation of passivation layers. Finally, these passivation layers can also protect the surface of current collector from corrosion, confirming that these new salts are effective additives in conventional electrolytes for high-voltage lithium ion batteries.
