Lithium malonatoborate additives enabled stable cycling of 5 V lithium metal and lithium ion batteries

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 LiNi_0.5Mn_1.5O₄ (LNMO) based lithium metal and lithium ion batteries. Cyclic voltammograms (CVs) reveal that the electrolytes with additives can significantly suppress the co-intercalation of solvents into the graphene layers during the first cycle due to their sacrificial reductions on the surface of the graphite electrode above 1.0 V vs Li/Li⁺. In addition, CVs reveal that the electrolyte without additive suffers from extensive electrolyte oxidation on the surface of the LNMO electrode during the first cycle, resulting in the biggest increase of the total cell impedance. Furthermore, electrochemical floating test shows less oxidation current in the electrolytes with additives at voltages above 5.0 V, proving good passivation by the additives. More importantly, the presence of additives can effectively increase the first cycle coulombic efficiencies and cycling stability in the LNMO based lithium metal and lithium ion batteries. Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) show that with additives compact solid electrolyte interphase (SEI) and thinner passivation layer are formed on the surfaces of the graphite and LNMO electrode, respectively. Finally, these salt additives can better protect the current collector from corrosion, further confirming their effectiveness in conventional electrolytes for high-voltage lithium metal and lithium ion batteries.