Sodium ion battery has attracted worldwide and intensive attention recently, while the adoption of electrolyte additives has been considered as one effective strategy to promote the cell performance. Within this work sodium difluoro(oxalato)borate (NaDFOB) was prepared and adopted as an additive for the general non-aqueous electrolyte formula of 1 mol·L-1 NaClO4/EC/PC (Vol: Vol=1:1), and the effects of the additive concentration on ionic conductivity and oxidization decomposition voltage were investigated in detail. In addition, the cell performance evaluated by NaNi0.5Mn0.5O2 as the cathode was also studied. It reveals that the addition of NaDFOB into the NaClO4/EC/PC electrolyte resulted in the significantly increased oxidation decomposition voltages from 4.6 V to 4.85 V, in spite of the slightly increased ionic conductivity, attributed to extra dissociated sodium salt NaDFOB. When the 0.025 mol·L-1 NaDFOB added electrolyte was used to support the operation of NaNi0.5Mn0.5O2 cathode, the initial irreversible capacity decreased from 22 mAh·g-1 to 9 mAh·g-1, and the capacity retention upon 200 cycles at 0.2 C-rate increased from 44.4% to 89.5%, with an average capacity fade of 0.06 mAh·g-1 per cycle. Therefore, the NaDFOB was proved to be an effective electrolyte additive for non-aqueous sodium ion batteries.