Insights into the improved cycle and rate performance by ex-situ F and in-situ Mg dual doping of layered oxide cathodes for sodium-ion batteries

P2 phase layered manganese-based transition metal (TM) oxides have been extensively studied as cathode materials in Sodium ion batteries (SIBs). Herein, ex-situ F-doping P2-type layered Na0.67Ni0.15Fe0.2Mn0.65F0.05O1.95 (NFMF-005) is synthesized firstly by coprecipitation to reduce the capacity fading caused by irreversible phase transition and Jahn-Teller effect. Because the bond energy of OO and TM-O are changed owing to O sites are occupied by F, leading to the decrease of the interatomic distance of TM and the increase of the interlayer spacing, NFMF-005 can provide wider ion transport channel and suppress structural deformation. To inhibit phase transition completely and mitigate the slide of TM layers, in-situ Mg-doping is secondly carried out by electrochemical method after F-doping of NFMF-005. The obtained Mg-doping P2-type NM-NFMF005 possesses outstanding electrochemical performance, delivering the specific capacity of 229 mAh g−1, the capacity retention of 87.7% after 50 cycles and discharge specific capacity of 100 mAh g−1 at 10 C. Mg-Mg dimers are used to explain its superior performance, which exhibit pillar effect not only to stretch the interlayer space, but also to enhance the mechanical strength and improve structure stability. All the results illustrate that the double-doping of F and Mg can play synergistic role to promote the performance of SIBs. This work is promising to further expand strategies and areas of materials doping. This new electrochemical doping method will also provide guidance for the rational design of SIBs layered cathode materials.