Co-Free Layered Oxide Cathode Material with Stable Anionic Redox Reaction for Sodium-Ion Batteries

Co shows excellent performance in the high voltage range for layered oxide cathode materials for sodium ion batteries (SIBs). However, its high cost and toxicity are significant disadvantages.Co-free cathodes with high performance are urgently needed for development. Herein, cheap Mg and Ti elements are preferred to replace Co elements. A P2-Na0.67Mn0.53Ni0.30Mg0.085Ti0.085O2 (Ni30MgTi) cathode shows a high reversible specific capacity of 118 mA h g(-1) at a current density of 50 mA g(-1) in 2.0-4.25 V, which is even higher than that in the base sample with Co. Moreover, Mg and Ti raise the median discharge voltage from 3.21 to 3.59 V, which raises the energy density from 325 to 410 Wh kg(-1). On the other hand, ex situ XPS and in situ differential electrochemical mass spectrometry tests indicate that Ni30MgTi has a stable anionic redox reaction with reversible capacity in the high voltage range. The concept of bimetallic co-substitution offers a simple and effective Co-free strategy to reduce the cost and increase the energy density of layered oxide cathode materials simultaneously for SIBs.