This work is aimed to synthesize anhydrous Li B H with high purity via solvent-free reaction between solid state reagents LiBH and B H . The reaction is conducted respectively in open system and sealed system for comparison. It is observed to experience three thermodynamic steps: (1) melting of B H at 103 °C, (2) phase transition of LiBH at 118 °C and (3) Li B H formation at 135 °C. With appropriate heat treatment at 180 °C for 2 h, Li B H with purity high as 93% is obtained in open system of 2LiBH  + 1.4B H with excess B H . Sealed system reacting at approximate even optimized conditions is unavailable to produce such high purity of Li B H . Chemical equilibrium confines the sufficient interaction between LiBH and B H in sealed system but it does not do in open system.