  Electrolyte decomposition reactions on Li-ion battery electrodes contribute
to the formation of solid electrolyte interphase (SEI) layers. These SEI layers
are one of the known causes for the loss in battery voltage and capacity over
repeated charge/discharge cycles. In this work, density functional theory
(DFT)-based ab-initio calculations are applied to study the initial steps of
the decomposition of the organic electrolyte component ethylene carbonate (EC)
on the (10-14) surface of a layered Li(Nix,Mny,Co1-x-y)O2 (NMC) cathode
crystal, which is commonly used in commercial Li-ion batteries. The effects on
the EC reaction pathway due to dissolved Li+ ions in the electrolyte solution
and different NMC cathode surface terminations containing absorbed hydroxyl -OH
or fluorine -F species are explicitly considered. We predict a very fast
chemical reaction consisting of an EC ring-opening process on the bare cathode
surface, the rate of which is independent of the battery operation voltage.
This EC ring-opening reaction is unavoidable once the cathode material contacts
with the electrolyte because this process is purely chemical rather than
electrochemical in nature. The -OH and -F adsorbed species display a
passivation effect on the surface against the reaction with EC, but the extent
is limited except for the case of -OH bonded to a surface transition metal
atom. Our work implies that the possible rate-limiting steps of the electrolyte
molecule decomposition are the reactions on the decomposed organic products on
the cathode surface rather than on the bare cathode surface.
