The structural and mechanical properties and stability nature of Li and Mg co-decorated planar eta-1/8 boron sheet have been investigated by density functional theory calculations. It is shown that the boron sheet is differently impressed by adding Li and Mg, and this doping process results in different geometric patterns originated from different binding nature of the adsorption cases. Interestingly, due to the planar (or in-plane) residual forces between the atoms, the stable intrinsic doping cases exist in addition to the external doping cases. The cohesive energies, binding and adsorption energies are calculated. The clustering energies of the possible dope cases are analyzed. The detailed bonding characteristics of the systems are presented by employing Mulliken atomic charges, bond overlap populations, and 2D/3D electron density maps. To investigate mechanical stability and to determine some mechanical properties such as layer modulus, Young's and shear moduli, and Poisson ratio, the elastic constants are calculated. Moreover, the multiple layer cases (2- and 3) of the boron sheet are studied with the van der Waals interactions, and then possible diffusion barriers for different paths are determined. The designed 2- or 3-boron layer systems could be promising cathode candidates for battery applications. (C) 2018 Elsevier B.V. All rights reserved.