  The intricate role of shear viscosity and ion-pair relaxations on ionic
conductivity mechanisms and the underlying changes induced by salt
concentration ($c$) in organic liquid electrolytes remain poorly understood
despite their widespread technological importance. Using molecular dynamics
simulations employing nonpolarizable force fields for $c$ ranging between
10$^{-3}$ to 101 M, we show that the low and high $c$ regimes of the EC-LiTFSI
electrolytes are distinctly characterized by $\eta\sim\tau_c^{1/2}$ and
$\eta\sim\tau_c^{1}$, where $\eta$ and $\tau_c$ are shear viscosity and
cation-anion relaxation timescales, respectively. Our extensive simulations and
analyses suggest a universal relationship between the ionic conductivity and c
as $\sigma(c)\sim c^{\alpha}e^{-c/c_{0}} (\alpha>0)$. The proposed relationship
convincingly explains the ionic conductivity over a wide range of $c$, where
the term $c^\alpha$ accounts for the uncorrelated motion of ions and
$e^{-c/c_0}$ captures the salt-induced changes in shear viscosity. Our
simulations suggest vehicular mechanism to be dominant at low $c$ regime which
transition into a Grotthuss mechanism at high $c$ regime, where structural
relaxation is the dominant form of ion transport mechanism. Our findings shed
light on some of the fundamental aspects of the ion conductivity mechanisms in
liquid electrolytes, offering insights into optimizing the ion transport in
EC-LiTFSI electrolytes.
