A high entropy O3-Na1.0Li0.1Ni0.3Fe0.1Mn0.25Ti0.25O2 cathode with reversible phase transitions and superior electrochemical performances for sodium-ion batteries

O3-type Ni/Mn/Fe-based layered oxides are very promising cathode materials for Na-ion batteries (SIBs) due to their high redox potential of Ni2+/Ni4+ and Jahn–Teller inactive centers (Ni2+, Fe3+, and Mn4+). However, their electrochemical performances are inferior for practical applications due to complex multi-phase transitions and structural instability at high voltages, Na+/vacancy ordering and transition-metal migration. Herein, we tailor a Li/Ti co-substituted high-entropy oxide (HEO)-Na1.0Li0.1Ni0.3Fe0.1Mn0.25Ti0.25O2 (NFMTO) cathode material, which exhibits highly reversible O3–P3–OP2 phase transitions with enhanced structural stability. Accordingly, the HEO-NFMTO cathode retains 88% and 77% of its initial capacity after 200 and 400 cycles at 1C and 2C rates, respectively. The co-substitution strategy also enhances Na+-ion diffusivity, resulting in good rate performances of NFMTO (102.5 and 78 mA h g−1 at 2C and 5C rates). A full Na-ion cell with an NFMTO cathode and hard carbon anode is fabricated, which retains 62% of its initial capacity even after 1000 cycles at 0.5C rate. The co-substitution strategy holds enormous potential for designing HEO cathode materials for practical SIBs.