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

초록

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.

키워드