Mitigating the P2-O2 transition and Na+/vacancy ordering in Na2/3Ni1/3Mn2/3O2 by anion/cation dual-doping for fast and stable Na+ insertion/extraction

P2-type Na2/3Ni1/3Mn2/3O2 is one of the most promising cathode candidates for sodium-ion batteries due to its high specific capacity and high working voltage. However, a detrimental P2-O2 phase transition usually occurs at a high voltage (>4.2 V) leading to poor cycle stability. Herein, we propose to mitigate this critical issue through a controlled F-/Ca2+ dual-doping strategy with CaF2 as a dopant. The divalent Ca2+ ion doped in the Na layer stabilizes the layered structure at a high voltage when excessive Na+ is extracted. The more electronegative F- ion forms a stronger transition metal (TM)-F bond and reduces the electrostatic repulsion between the oxygen layers impeding the gliding of TMO2 layers. The Ca2+/F- co-doping successfully suppresses the unfavorable P2-O2 phase transition, and significantly improves the structural stability and cycling performance (27.1% vs. 87.2% after 500 cycles at 1C). Furthermore, density functional theory calculations combined with experimental tests reveal that the incorporation of Ca2+ and F- in Na sites and O sites facilitates the electronic and ionic conductivity owing to Na+/vacancy disordering, which enhances the high-rate capability. This study provides some insights into the design of long-life and high-rate cathode materials for sodium-ion batteries.

Automated summary

In this study, a controlled F-/Ca2+ dual-doping strategy was proposed to mitigate the P2-O2 phase transition, significantly improving the structural stability and cycling performance. The doping of Ca2+ and F- facilitated electronic and ionic conductivity, enhancing the high-rate capability.