P2-Na0.6[Cr0.6Ti0.4]O2 cation-disordered electrode for high-rate symmetric rechargeable sodium-ion batteries

Most P2-type layered oxides exhibit Na(+)/vacancy-ordered superstructures because of strong Na(+)-Na(+) interaction in the alkali metal layer and charge ordering in the transition metal layer. These superstructures evidenced by voltage plateaus in the electrochemical curves limit the Na(+) ion transport kinetics and cycle performance in rechargeable batteries. Here we show that such Na(+)/vacancy ordering can be avoided by choosing the transition metal ions with similar ionic radii and different redox potentials, for example, Cr(3+) and Ti(4+). The designed P2-Na(0.6)[Cr(0.6)Ti(0.4)]O2 is completely Na(+)/vacancy-disordered at any sodium content and displays excellent rate capability and long cycle life. A symmetric sodium-ion battery using the same P2-Na(0.6)[Cr(0.6)Ti(0.4)]O2 electrode delivers 75% of the initial capacity at 12C rate. Our contribution demonstrates that the approach of preventing Na(+)/vacancy ordering by breaking charge ordering in the transition metal layer opens a simple way to design disordered electrode materials with high power density and long cycle life.