  Using density functional theory combined with a semi-empirical van der Waals
dispersion correction, we have investigated the stability of lattice defects
including boron vacancy, substitutional and interstitial X (X=H, C, B, N, O)
and $\Sigma$5 tilt grain boundaries in borophene and their influence on the
anisotropic mechanical properties of this two-dimensional system. The pristine
borophene has significant in-plane Young's moduli and Poisson's ratio
anisotropy due to its strong and highly coordinated B-B bonds. The
concentration of B vacancy and $\Sigma$5 grain boundary could be rather high
given that their formation energies are as low as 0.10 eV and 0.06 eV/$\AA$
respectively. In addition, our results also suggest that borophene can react
easily with H$_2$, O$_2$ and N$_2$ when exposed to these molecules. We find
that the mechanical properties of borophene are remarkably degraded by these
defects. The anisotropy in Poisson's ratio, however, can be tuned by some of
them. Furthermore, the adsorbed H or substitutional C may induce remarkably
negative Poisson's ratio in borophene, and the substitutional C or N can
significantly increase the Poisson's ratio by contrast.
