Enhanced electrochemical performance of Ti substituted P2-Na2/3Ni1/4Mn3/4O2 cathode material for sodium ion batteries

• Ti substituted P2-Na2/3Ni1/4Mn3/4O2 cathode was synthesized.
• Structural and electrochemical properties of Na2/3Ni1/4TixMn3/4-xO2 were studied.
• Ti substituted cathodes exhibit enhanced cycleability and rate performance.
• Ti substitution has impact on stabilizing the P2 structure during cycling.

Ti substituted P2-Na2/3Ni1/4Mn3/4O2 cathode material with the composition of Na2/3Ni1/4TixMn3/4-xO2 has been synthesized by solid state method. The influence of Ti substitution for Mn on the structure, morphology and electrochemical performances of P2-Na2/3Ni1/4Mn3/4O2 has been investigated. X-ray diffraction (XRD) results of Ti substituted sample show that they exhibit same diffraction patterns as those of pristine P2-Na2/3Ni1/4Mn3/4O2. Progressive change in the lattice parameters of Ti substituted samples suggests that Mn was successfully substituted by Ti. In contrast to P2-Na2/3Ni1/4Mn3/4O2 which shows step-type voltage profiles, Ti substituted samples show sloping voltage profiles. Drastic capacity fade occurred for P2-Na2/3Ni1/4Mn3/4O2 cathode, while Ti substituted cathodes still show high capacity retention over 92% after 25 cycles at the voltage range of 2.0-4.3 V. Even cycled at high upper cut-off voltage of 4.5 V, Ti=0.20 sample can deliver a reversible capacity of 140 mAhg−1 with the capacity retention over 92% after 25 cycles. Furthermore, Ti substituted cathodes exhibit enhanced rate capability over pristine P2-Na2/3Ni1/4Mn3/4O2 cathode. Comparison of the Ex-situ XRD results of the cycled P2-Na2/3Ni1/4Mn3/4O2 and its substituted samples provides evidence that the improved electrochemical performance of Ti substituted cathodes would be attributed to the stabilization of the structure with Ti substitution.