Improving the Crystal Stability of O3-type Metal Oxide Cathode Materials by Cu Substitution for Na-Ion Batteries

Nickel–iron–manganese-based (NiFeMn-based) O3-type layered oxides hold substantial promise as cathode materials for large-scale applications in sodium-ion batteries (SIBs). However, their cyclic stability still requires optimization by elemental substitution at different lattice sites. Herein, we present a high-temperature solid-state reaction synthesis of a pure O3-phase NaNi0.3Cu0.1Fe0.2Mn0.4O2 (NCFM) exhibiting a reversible capacity of 126.5 mAh g–1 and maintaining a capacity retention of 89% after 100 cycles at 0.2 C. Upon continuous charging and discharging, the discharge plateaus initially situated at 2.6 V gradually transform into a sloped curve ranging from 2.8 to 2.6 V. The noteworthy improvement in cyclic stability observed in NCFM underscores the significant impact of Cu substitution in NiMnFe-based layered oxide materials for SIBs. Ex situ X-ray diffraction indicates that NCFM undergoes a reversible "O3–P3–O3 + P3–O3" phase transition during cycling. Our findings highlight the advantageous role of Cu substitution in NCFM, showcasing its potential as a promising cathode material for enhancing the performance of SIBs.