  The porosities and tortuosities are commonly utilized to characterize the
microstructure of a Li-ion battery's separator and are adopted as key input
parameters in advanced battery models. Herein, a general classification of the
tortuosity for a porous medium is introduced based on its bi-fold significance,
i.e., the geometrical and physical tortuosities. Then, three different methods
for the determination of separator's electrical tortuosity are introduced and
compared, which include the empirical Bruggeman equation, the experimental
method using Electrochemical Impedance Spectrum (EIS) testing a the numerical
method using realistic 3D microstructure of the separator obtained from
nanoscale X-ray Computed Tomography (XCT). In addition, the connection between
the geometrical tortuosity and the electrical tortuosity of a separator is
established by introducing the electrical phenomenological factor (\b{eta}_e),
which can facilitate the understanding of the relationship between the
microstructure characteristics and transport properties of the separators.
Furthermore, to quantitively compare the values of the tortuosities determined
by different methods, the corresponding effective transport coefficients
({\delta}) are compared, which was usually used as a correction for effective
diffusivity and conductivity of electrolytes in porous media.
