Structural and Electrochemical Consequences of Sodium in the Transition-Metal Layer of O′3-Na₃Ni_1.5TeO₆

Sodium layered oxide cathodes for rechargeable batteries suffer from Na+ ordering and transition-metal layer gliding that lead to several plateaus in their voltage profile. This characteristic hinders their competitiveness as a viable option for commercial rechargeable batteries. In O′3-layered Na3Ni1.5TeO6 (Na5/6[Na1/6Ni3/6Te2/6]O2), Rietveld refinement and solid-state nuclear magnetic resonance spectroscopy show that there is sodium in the transition-metal layer. This sodium within the transition-metal layer provides cation disorder that suppresses Na+ ordering in the adjacent sodium layers upon electrochemical insertion/extraction of sodium. Although this material shows a reversible O′3 to P′3 phase transition, its voltage versus composition profile is typical of traditional lithium layered compounds that have found commercial success. A Ni2+/3+ redox couple of 3.45 V versus Na+/Na is observed with a specific capacity as high as 100 mAh g–1 on the first discharge at a C/20 rate. This material shows good retention of specific capacity, and its rate of sodium insertion/extraction can be as high as a 2C-rating with particle sizes on the order of several micrometers. The structural nuances of this material and their electrochemical implications will serve as guidelines for designing novel sodium layered oxide cathodes.