Completely suppressed high-voltage phase transition of P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2via Li/Ni co-doping for sodium storage

P2-type Fe, Mn-based layered oxides have been potential cathode materials for sodium-ion batteries (SIBs), yet suffer from their intrinsic sluggish kinetics and structural instability due to the adverse P2–Z high-voltage phase transition. An improvement strategy by either single-cation doping or co-doping is used, but typically with either a lowered reversible capacity or partially suppressed high-voltage phase transition. In this study, a novel biphasic P2/O3-Na0.7Li0.1Ni0.1Fe0.2Mn0.6O2 cathode was prepared, with the high-voltage phase transition completely suppressed via Li/Ni co-doping. Inactive Li+ stabilizes the structure and active Ni2+ improves the electrical conductivity, while the P2/O3 intergrown structure induced by co-doping further limits the lattice stress during cycling. The resulting cathode exhibits an outstanding rate capability (102.2 mA h g−1 at 0.1C and 59.8 mA h g−1 at 10C), and an excellent cyclic stability (74.6% capacity retention after 500 cycles at 10C). The reaction kinetics and structural evolution demonstrate high Na+ diffusion coefficient and the complete suppression of the Z phase transition, respectively, both of which underpin the enhancement. The results highlight that the synergistic effect between Li/Ni co-doping and accompanying biphasic structure promises an effective improvement strategy to develop high-performance Fe, Mn-based and Co-free layered cathode materials for SIBs.