Dendrite formation is a long-standing issue in lithium metal batteries. Replacing the conventional liquid electrolytes with semi-solid ones, the non-uniform lithium growth can be potentially mitigated by the mechanical deformation in the solid matrix. The underlying dendrite suppression mechanism is investigated in this study using a mechano-electrochemical phase-field method. Two indicators, namely the arithmetic average height and the elongation rate, are proposed to characterize the surface roughness of lithium dendrites. Our simulation results are summarized in two-dimensional design maps as a function of the porosity and the elastic modulus of the semi-solid electrolytes, which could provide us the guidance for the development of dendrite-free lithium metal batteries.