This study reports on solid-state batteries operating at 120 °C prepared with metal hydride MH nanocomposites xMgH2+(1-x)TiH2 used as active materials for the positive electrode, metallic Li as the negative electrode and LiBH4 as the solid electrolyte. The molar content x of the MH nanocomposites ranged from 0.2 to 0.8. The electrochemical properties of the metal hydride electrodes are here scrutinized as a function of the chemical composition of the active materials. Mg-rich nanocomposites offer higher specific capacity (over 1700 mAh g−1 at C/50 regime for x = 0.7), whereas Ti-rich ones exhibit better cycle-life (ca. 100% capacity retention after 10 cycles for x = 0.2). In comparison to equivalent liquid electrolyte cells operated at RT, solid-state cells show improved properties such as a coulombic efficiency above 98% and a rate capability of ca. 50% of delivered capacity at 1C-rate. It is also evidenced that mass transport within the working electrode is a key factor limiting the reversibility of hydride conversion reactions. This work represents a deep insight on the hurdles of metal hydrides to be used as efficient anode materials for Li-ion batteries.