In 21^th century, rechargeable batteries are main key of modern technology in many applications from portable devices (smartphone, laptop…) to large-scale (hydride electric vehicle-HEV, smart grid system…). Among the rechargeable batteries, Li-ion battery (LIB) is outstanding member due to the highest gravimetric as well as volumetric capacity; and Sodium-ion batteries (SIBs) can have contribution to alternating LIBs in large-scale application. Li-ion and Na-ion batteries have the same configuration with an insertion/extraction reversible of Li⁺ ions and Na⁺ ions into electrode positive and negative during charge-discharge process. This work aimed to investigate the effect of Ni-doping on the electrochemical performance of olivine LiFePO₄ in sodium secondary batteries. The Ni-doping olivines LiFe_1-xNi_xPO₄ (0 < x ≤ 0.25) were prepared by hydrothermal route which exhibited a high crystallization and good distribution grains in nanometric scale. The delithiated phases Fe_1-xNi_xPO₄ were obtained by the electrochemical oxidation at rate C/20. In charge-discharge profile in Na-cell, a specific capacity was found out 125 mAh/g with a capacity retention of 95% after 50 cycles.