Boron (B) has a high theoretical lithium storage capacity of 3046mAhg−1 for forming Li5B4 phase. Li–B alloy has been used in high temperature thermal lithium batteries. In this work, a new tetragonal boron (B50) thin film with a thickness of 80nm has been deposited on a vanadium coated glass substrate by a pulse laser deposition (PLD) method. It is found that this electrode film shows only a lithium storage capacity of 43mAhg−1 in a nonaqueous electrolyte at room temperature. According to a first-principles calculation, the B50 is a metallic conductor. Therefore, the very poor activity could be related to poor lithium ion diffusion property since the diffusion barrier in this tetragonal B50 lattice is calculated as 2.59eV. This consists well with the fact that the lithium storage capacity in the B50 thin film is improved significantly to 268mAhg−1 at 85°C.