P3/O3 Integrated Layered Oxide as High-Power and Long-Life Cathode toward Na-Ion Batteries

Abstract

Low-cost and stable sodium-layered oxides (such as P2- and O3-phases) are suggested as highly promising cathode materials for Na-ion batteries (NIBs). Biphasic hybridization, mainly involving P2/O3 and P2/P3 biphases, is typically used to boost their electrochemical performances. Herein, a P3/O3 intergrown layered oxide (Na_2/3 Ni_1/3 Mn_1/3 Ti_1/3 O₂ ) as high-rate and long-life cathode for NIBs via tuning the amounts of Ti substitution in Na_2/3 Ni_1/3 Mn_{2/3- x} Ti_x O₂ (x = 0, 1/6, 1/3, 2/3) is demonstrated. The X-ray diffraction (XRD) Rietveld refinement and aberration-corrected scanning transmission electron microscopy show the co-existence of P3 and O3 phases, and density functional theory calculation corroborates the appearance of the anomalous O3 phase at the Ti substitution amount of 1/3. The P3/O3 biphasic cathode delivers an unexpected rate capability (≈88.7% of the initial capacity at a high rate of 5 C) and cycling stability (≈68.7% capacity retention after 2000 cycles at 1 C), superior to those of the sing phases P3-Na_2/3 Ni_1/3 Mn_2/3 O₂ , P3-Na_2/3 Ni_1/3 Mn_1/2 Ti_1/6 O₂ , and O3-Na_2/3 Ni_1/3 Ti_2/3 O₂ . The highly reversible structural evolution of the P3/O3 integrated cathode observed by ex situ XRD, ex situ X-ray absorption spectra, and the rapid Na⁺ diffusion kinetics, underpin the enhancement. These results show the important role of P3/O3 biphasic hybridization in designing and engineering layered oxide cathodes for NIBs.

Keywords: Na-ion batteries; P3/O3 biphase; cathode; electrochemistry; layered oxides.