Preserving reversible transition metal redox centers in the O3-type layered oxide cathodes for high-energy sodium-ion batteries

Abstract

The O3-type Na[Ni1/3Fe1/3Mn1/3]O2 (O3-NFM) cathode has attracted considerable attention for sodium-ion batteries (SIBs) owing to its reversible Na+ storage and low cost. However, the practical application of the O3-NFM cathode is limited by insufficient energy density. In this study, through the regulation of crystal and electronic structures, we design the high-energy density O3-Na0.94Ca0.03[(Ni1/3Fe1/3Mn1/3)0.99Al0.01]O2 (O3-CNFMA) cathode. In theory, the incorporation of Ca2+ and Al3+ suppresses transition metal (TM) migration into Na layer and irreversible oxygen reaction while promoting TM-centered redox behavior. Ca2+ in the Na layer act as a pillar, mitigating lattice strain during deep Na extraction. Al strengthens TM-O bonding and widens Na slab spacing, enhancing Na+ kinetics. Furthermore, partial Al3+ substitution disrupts continuous TM-O-TM superexchange pathways and weakens long-range magnetic connectivity, which increases the energetic barrier for cation rearrangement and restrains irreversible Fe migration under deep charging operation to 4.2 V (vs. Na/ Na+). In the voltage window of 2.0-4.2 V, the O3-CNFMA cathode delivers 175.5 mAh g-1 with 99.1% Coulombic efficiency, excellent cycling stability, and power capability without noticeable microcracking. Moreover, it exhibits strong water resistance and long-term cycling stability in a hard carbon-based pouch-type full cell.