Investigation of the Electrode-Electrolyte Interphase in Ester-Based Electrolytes in NCM523/Graphite Cells

Abstract

The use of methyl acetate (MA) and methyl propionate (MP) as co-solvents in carbonate/LiPF 6 electrolytes have been investigated in LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523)/graphite cells to improve electrochemical performance at low temperature (−20 °C). Consistent with previous investigations, improved low temperature performance was observed with all electrolyte formulations containing esters. Detailed ex situ surface analysis of the surface films generated with the ester-based electrolytes containing electrolyte additives reveal significant changes to the SEI on graphite upon cycling at moderately elevated temperature (45 °C). In the presence of vinylene carbonate (VC) lithium plating is observed upon cycling at low temperature (−20 °C) which leads to a thicker solid electrolyte interface (SEI) upon cycling at 45 °C. Incorporation of fluoroethylene carbonate (FEC) as an electrolyte additive results in the generation of a thin SEI and alters the structure of the surface film on the NCM523 cathode resulting in stable cycling over a wide temperature range.