  The strategy of band convergence of multi-valley conduction bands or
multi-peak valence bands has been widely used to search or improve
thermoelectric materials. However, the phonon-assisted intervalley scatterings
due to multiple band degeneracy are usually neglected in the thermoelectric
community. In this work, we investigate the (thermo)electric properties of
non-polar monolayer $\beta$- and $\alpha$-antimonene considering full mode- and
momentum-resolved electron-phonon interactions. We also analyze thoroughly the
selection rules on electron-phonon matrix-elements using group-theory
arguments. Our calculations reveal strong intervalley scattering between the
nearly degenerate valley states in both $\beta$- and $\alpha$-antimonene, and
the commonly-used deformation potential approximation neglecting the dominant
intervalley scattering gives inaccurate estimations of the electron-phonon
scattering and thermoelectric transport properties. By considering full
electron-phonon interactions based on the rigid-band approximation, we find
that, the maximum value of the thermoelectric figure of merits $zT$ at room
temperature reduces to 0.37 in $\beta$-antimonene, by a factor of 5.7 comparing
to the value predicted based on the constant relaxation-time approximation
method. Our work not only provides an accurate prediction of the thermoelectric
performances of antimonenes that reveals the key role of intervalley
scatterings in determining the electronic part of zT, but also showcases a
computational framework for thermoelectric materials.
