  The lattice thermal conductivity and thermal transport properties of 2D
$\alpha$-selenene are investigated based on the first-principles calculations.
The isotropic in-plane thermal conductivity is as low as 3.04 W m$^{-1}$
K$^{-1}$ at room temperature, even abnormally lower than $\alpha$-tellurene
which processes analogous configuration and lower Debye temperature. We find
this abnormal phenomenon reasonably stems from the larger anharmonicity of the
acoustic phonon branch. Moreover, the phonon spectra, elastic properties, and
related thermal properties are also exhibited. Acoustic phonons contribute
mainly to the total thermal conductivity. Furthermore, size effect, boundary
effect, the total phase space for three-phonon processes, phonon group velocity
and relaxation time are further investigated, and the last one is unveiled to
be the key ingredient of thermal transport in 2D selenene.
