Elevating both capacity and voltage tolerance of P2-type layered cathodes with cooperative Al cation/F anion co-doping for advanced sodium-ion batteries

The single ionic doping helps suppress the detrimental phase transition when P2-type layered cathodes are charged to a high voltage above 4.0 V (vs. Na+/Na). However, this is realized at the sacrifice of their electrochemical redox centers and output capacities. To achieve both high voltage tolerance and capacity, we herein propose a cooperative Al cation and F anion co-doping strategy. The XANES detections affirm that substituting Ni/O with Al/F intends to augment the amount of highly active Mn3+ cations in cathodes, making more contributions on specific capacities for sodium-ion batteries (SIBs). Besides, this co-doping treatment would disorder the transition metal ionic arrangements of cathodes, disrupting long-range Jahn-Teller effects and impeding other undesired phases generation. As a proof-of-concept demonstration, our designed Na2/3Ni0.23Al0.1Mn2/3O1.95F0.05 (NAF) cathodes show a delivered capacity as high as 142.0 mAh g−1 (0.2C), and an impressive capacity retention of 86.7 % after all cycling. The in-situ XRD detections reveal no apparent O2 phase peaks emerge until 4.23 V upon deep Na+ extraction from NAF. This work provides a key understanding toward cation/anion co-doping effects, opening up a useful avenue for rational design of practical P2-type cathodes for SIBs.