Ni2+-doped Na2Ti6O13 nanotubes to enhance the electrochemical performance of sodium ion batteries

•Ni2+-doped Na2Ti6O13 nanotubes was prepared with hydrothermal combined ultrasonic method.•The net-like structure composed of nanotubes increased the contact area and the reactive site.•Ni2+ doping resulted in ion diffusion rate and cycle stability of the materials.•The characteristic oxidation potential of Na2Ti6O13 has been reduced.•The specific capacity of Ni2+-doped Na2Ti6O13 increased by 41 %.