Enhanced sodium storage characteristics of P2-Na2/3Mn3/4Co1/4O2 cathode co-modified by La2O3 and TiO2 oxide

Abstract

A sodium-rich P2-Na2/3Mn3/4Co1/4O2 cathode is co-modified by La2O3 and TiO2 oxide through a solid-state method. Rietveld refinement confirms that a few La3+ and Ti4+ ions have substituted the Na sites and the TM sites in the crystal lattice of P2-Na2/3Mn3/4Co1/4O2, respectively, which leads to the enlargement of Na–O bond length and Na+ migration channels, and the shrinkage of TM–O and O–O bonds. Surface co-modification has reduced Na+ migration resistance, increased Na+ diffusion coefficient and stabilized crystal structure, which can significantly enhance its rate capability and especially cycle stability. The modified layers have been formed on the surface of the Na2/3Mn3/4Co1/4O2, which is capable of preventing the Na2/3Mn3/4Co1/4O2 oxide from exfoliation caused by the surface reactions in the electrode/electrolyte interfaces. It will help to stabilize the layered structure, alleviate Jahn–Teller distortion and form a thinner interface film among the electrode/electrolyte. The optimum modified cathode oxide is prepared using 1 wt% composite oxide in which the mass ratio of La2O3 and TiO2 is 1:1. In the voltage range of 1.8–4 V, the optimum modified cathode delivers a maximum areal capacity of 164.3 mAh g−1 cm−2 at 0.1 C with the capacity retention of 89.5% at 50th cycle and 81.7% after 100 cycles. When cycling at 10 C, it can retain 90.1% of its maximum 79.9 mAh g−1 cm−2 at 240th cycle, and its capacity retention is up to 80.2% even after 400 cycles. The excellent cycle stability, especially superior high-rate cycle stability, can still be traced back to the dual effects of the surface modification and the substitution of La3+ or Ti4+ ions. The high-performance cathode material co-modified by the La2O3 and the TiO2 oxide will give impetus to the development and research of cathode layered oxides for SIBs.