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

The use of methyl acetate (MA) and methyl propionate (MP) as co-solvents in carbonate/LiPF6 electrolytes have been investigated in LiNi0.5Co0.2Mn0.3O2 (NCM523)/graphite cells to improve electrochemical performance at low temperature (-20 degrees 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 degrees C). In the presence of vinylene carbonate (VC) lithium plating is observed upon cycling at low temperature (-20 degrees C) which leads to a thicker solid electrolyte interface (SEI) upon cycling at 45 degrees 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.