Superior Na-ion storage achieved by Ti substitution in Na3V2(PO4)3

The development of sodium-ion batteries (SIBs) remains a great challenge due to poor stability and sluggish kinetics of cathode materials. Here, we present Ti-substituted Na3-xV2-xTix(PO4)(3)/C (NVP-Ti-x/C, 0 <= x <= 0.2) as high-performance cathode materials for SIBs. Crystal structure and Na storage properties are investigated by experiment and theoretical calculation. X-ray diffraction results reveal that NVP-Ti-x have same rhombohedral structure as NVP but shrunk lattice parameters. Pair distribution function spectra and the calculated crystal structure suggest that the local structural distortion of the doped samples originates from slight decrease of V-O bond. As SIB cathode materials, greatly enhanced structural stability and electrochemical performance are achieved by Ti substitution. Especially, NVP-Ti-0.15/C exhibits excellent rate capability and cyclability with capacity retention of 60% after 2000 cycles at high rate of 10 C. Importantly, even at 20 C, a reversible capacity of 63 mAh g(-1) is maintained after 300 cycles at elevated temperature of 60 degrees C. Density functional theory calculations demonstrate that the superior electrochemical performance of the Ti-doped samples can be attributed to the enhanced intrinsic electronic conductivity, low Na+ diffusion energy barrier and high structural stability.