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 material with the composition of Na(2/3)Ni(1/4)TixMn(3/4-x)O(2) 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-Na2/3Ni(1/4)Mn(3/4)O(2) 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. (C) 2015 Elsevier Ltd. All rights reserved.