We report a strategy of using small cations to construct ionic liquid octahydrotriborate. The novel liquid Li(NH3)B3H8, prepared by a facile reaction of NH4B3H8 with LiH, froze below -33.4 °C and crystallized into a monoclinic unit cell with lattice parameters of a = 8.813(1) Å, b = 8.8626(1) Å, c = 8.2076(1) Å, and β = 110.1046(5)°, providing a promising functional liquid octahydrotriborate with the highest B3H8- content and lowest freezing temperature.