This paper presents the first phase of work on cement design for energy storage wells. To achieve tough cementitious materials capable of withstanding frequent thermal shocks and providing thermal insulation this phase involved formulation of lightweight cements, using foam and cenospheres technologies, and evaluation of their mechanical properties after 24-hr curing at target temperatures (100, 175 and 250oC). This was followed by the cement modifications with the waterborne polymers, its reinforcement with polymer fibers and re-evaluation for the mechanical properties. The two tested technologies, air foaming and cenospheres offered formulations with a wide range of densities. The foam allowed lower density slurries, all the way down to 0.9SG. The mechanical properties of the TILTSRC, measured after 24hr at the target temperature, depended on the slurry density and the curing temperature. All tested slurries with densities below ~1.1SG did not meet 500 psi compressive strength after 24hrs requirement. High-temperature stable polymers and polymer fibers (polyvinyl alcohol and nylon) were tested to improve mechanical properties of the lightweight slurries. Among the tested polymers the carboxylated acrylic-modified lightweight cements performed the best improving slurries and cement properties such as lowering water-to-cement ratio and slurry densities, increasing workability, and compressive strength at temperatures of 175 and 250oC. Nylon fibers at 1% bwob offered the best reinforcement of TILTSRC with cenospheres increasing not only compressive strength but, more importantly, compressive toughness of the cement without compromising slurries mixing properties. Polymer- and fiber-modifications are expected to improve cement insulating properties that will be measured in the next phase of the work.