Na_2/3Li_1/9[Ni_2/9Li_1/9Mn_2/3]O₂: A High-Performance Solid-Solution Reaction Layered Oxide Cathode Material for Sodium-Ion Batteries

Designing high-performance layered cathodes, in particular the P2-type groups, is still challenging for sodium-ion batteries. Taking advantage of the well-known Li/Ni mixing in Ni-rich layered cathodes for Li-ion batteries, a layered Na2/3Li1/9[Ni2/9Li1/9Mn2/3]O2 cathode with a superior rate and long-cycle performance is developed through Li-ion incorporation. Neutron diffraction indicates that the introduced Li ions occupy the intrinsic Na+ vacancies in the Na layer and transition metal layer concurrently. A facile solid-solution reaction mechanism upon cycling is also revealed by in situ X-ray diffraction, and ex situ X-ray photoelectron spectroscopy results demonstrate that the Ni2+/Ni4+ pairs participate in the electrochemical reactions, while the Mn ions remain unchanged. This material delivers ∼91.6% of the theoretical capacity initially and 64 mAh g–1 reversible capacity even at 20 C (∼58.2% of the capacity at 0.1 C) with 74.5% capacity retention after 1500 long-term cycles in the voltage range of 2.0–4.2 V. This approach can optimize the material structure design, providing some insights into designing high-rate and long-cycle life oxide cathodes for sodium-ion batteries.