Rational design of Na-content adjustable layered oxide materials with tunable P2/O3 biphase for advanced sodium-ion cathodes

The role of Na-content on the structural stability and electrochemical performance of P2/O3-type cathode was established. • The structural stability of P2/O3-type Na x Ni 0.23 Mn 0.57 Fe 0.1 Cu 0.1 O 2 cathode is improved by Cu/Fe co-doping. • High Na-content improves the Na⁺ diffusion coefficient as well as rate performance of Na x Ni 0.23 Mn 0.57 Fe 0.1 Cu 0.1 O 2 cathode. P2/O3 biphasic layered oxide cathodes combine the structural stability and fast Na + transport of the P2 phase with the high Na + storage capacity, high operating voltage of the O3 phase. In order to deeply investigate the effect of P2/O3 biphasic composition on the electrochemical performance, Na x Ni 0.23 Mn 0.57 Cu 0.1 Fe 0.1 O 2 samples (NMCF- x ) with the tunable P2/O3 biphasic composition were prepared. The X-ray diffraction results show that the relative ratio of O3 phase increases from 48.53% to 84.46% with increasing 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 0.2C and 5C, respectively) and high capacity retention (69.8% after 500 cycles at 5C). The galvanostatic intermittent titration analysis confirms that the Na + diffusion coefficient reaches as high as 6.99 × 10 –11 cm 2 s –1 . These excellent performances are ascribed to the fact 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. Cu/Fe co-doping P2/O3 hybrid layered oxide cathodes for SIBs have been prepared through adjusting Na-content, revealing the influence of P2/O3 composition to cycling performance. Download: Download high-res image (248KB) Download: Download full-size image