Rational Design of Na-Content Adjustable Layered Oxide Materials with Tunable P2/O3 Biphase for Advanced Sodium-Ion Cathodes

Abstract

P2/O3 biphasic layered oxide cathode possesses both high Na+ storage capacity and structural stability of P2-phase, and high operating potential and air stability of O3-phase. In order to deeply investigate the effect of P2/O3 biphasic composition on and electrochemical performance, NaxNi0.23Mn0.57Cu0.1Fe0.1O2 samples (NMCF-x) with the tunable P2/O3 biphasic composition were prepared via coprecipitation and solid-phase method. The PXRD results by Rietveld refinement show that the relative ratio of O3 phase increases from 48.53% to 84.46% as rising Na content from x=0.70 to x=0.85. The O3 phase-rich NMCF-0.85 cathode exhibits high rate performance (130.1 and 94.6 mAh g-1 at 0.2 C and 5 C, respectively) and high capacity retention (69.8% after 500 cycles at 5 C). The galvanostatic intermittent titration analysis confirm Na+ diffusion coefficient as high as 6.99 × 10-11 cm2 s-1. These excellent performances are ascribed to that Cu/Fe co-doping and high Na content suppress the unfavorable P2-O2 phase transition and Na+/vacancy ordering in the NMCF-0.85 when charged to high potential. Thus, this finding can provide profound implications for developing stable layered oxide cathode materials with adjustable P2/O3 biphase for high-energy Na+ storage.

Keywords: sodium ion battery, layered oxide cathode, P2/O3 biphase, Cu/Fe co-doping, long-term cyclability