  It is shown that continuously occurring electrolyte-reduction reaction on
freshly-exposed electrode surfaces during lithiation/delithiation cycles causes
the lowering of cycling efficiency, and hence, capacity fade in well-cycled
silicon anodes in lithium-ion batteries. Using galvanostatic
lithiation/delithiation data from multiple cycles on a Li/Si half-cell, a
methodology to separate the charge due to the main reaction
(lithiation/delithiation of Si) from the side-reaction (electrolyte-reduction)
is presented. The rate of this parasitic side reaction is estimated on
well-cycled amorphous silicon thin-film electrodes at ambient temperature for
the following three commonly-used lithium-ion electrolyte formulations:
mixtures of ethylene carbonate and diethylene carbonate (EC:DEC) with and
without a fluoroethylene carbonate (FEC) additive, and propylene carbonate
(PC), all containing 1.2 M lithium hexafluorophosphate. Among the three
formulations, the electrolyte containing EC:DEC with the FEC additive exhibits
the lowest coulombic losses due to side-reactions, followed by PC, and EC:DEC
without the FEC additive (i.e., EC:DEC + FEC > PC > EC:DEC). The importance of
estimating side-reaction rates on a well-cycled electrode is discussed in the
context of self-discharge, capacity fade, development of battery management
system algorithms and precise mathematical modeling of lithium-ion batteries.
