The electronic structure, lattice dynamics and transport properties show significant anisotropy, which arises from the unique quasi-layered crystal structure. The anisotropy presents most clearly in the electronic band structure, where the flat bands along the real-space z direction inhibit the electrical conductivity and lead to a ZT that is two orders of magnitude lower compared to the in-plane figure of merit. This is despite the lower lattice thermal conductivity, which is predicted to be ∼30% smaller along the out-of-plane direction than in the xy plane.