With high hydrogen capacity, lithium borohydride (LiBH ₄, LBH) is considered as a promising hydrogen storage material. However, high dehydrogenation temperature (460 °C) and critical rehydrogenation conditions (155 bar H ₂ at 600 °C), hindered by the stable Li ₂B ₁₂H ₁₂, downgrade the LBH application in hydrogen storage. In this research, we investigated the nanoconfinement effect of LBH with lithium aluminum hydride (LiAlH ₄, LAH, also called lithium alanate) and magnesium chloride (MgCl ₂), forming a ternary system LiBH ₄–LiAlH ₄–MgCl ₂ (borohydride, alanate, and magnesium chloride; BAM) in different porous carbon supports. BAM confined in activated carbon (BAM@AC) showed the lowest peak dehydrogenation temperature at 227 °C, with a 98.6% hydrogen generation yield. The activation energy of LBH decomposition decreased tremendously from 146 to 59 kJ/mol in BAM@AC with reaction path modification. For rehydrogenation, 93.71% of LBH was regenerated in BAM@AC at 350 °C under 30 bar H ₂. The improved LBH rehydrogenation is due to the formation of MgAlB ₄ after BAM thermolysis, which represented the manipulation of the reaction route and the prohibition of Li ₂B ₁₂H ₁₂ formation. The much lower dehydrogenation temperatures and mild regeneration conditions in lithium borohydride show a promising perspective in the future development of complex metal hydrides in hydrogen storage.