Substituted Dioxaphosphinane as an Electrolyte Additive for High Voltage Lithium-Ion Cells with Overlithiated Layered Oxide

Abstract

4-methyl-2-[(2,2,3,3,4,4,5,5-octafluoropentyl)oxy]-1,3,2-dioxaphosphinane with the oxidation number of phosphorous (III) is used as an oxidative additive (OA) to a standard carbonate-based electrolyte for the high-voltage Li-ion cells with the overlithiated layered oxide Li1.20Ni0.18Mn0.53Co0.09O2 (OLO) as a positive electrode. Electrochemical stability of electrolytes with and without OA is compared by linear sweep voltammetry, and characteristics of coin half and full cells are examined by means of cycling tests and electrochemical impedance spectroscopy. Presence of OA in electrolyte mixture provides noticeable improvement in Coulombic efficiency, capacity retention, and rate properties of the cells, most likely, through the formation of an interface layer on the OLO-surface due to the decomposition of OA. Morphology of OLO after cycling with OA-containing electrolyte is investigated by scanning electron microscopy and the presence of amorphous coating is observed; 31P NMR analysis reveals that the products by the oxidation of OA are present on the cathode's surface. Differential scanning calorimetry data point out the substantially improved thermal stability of the OLO cathode after cycling in OA-containing electrolyte. Therefore, substituted dioxaphosphinanes may be considered as a promising structural pattern for design of new additives for the development of high-voltage electrolytes.