LiCoO 2 is a dominant cathode material for Li-ion batteries due to its high volumetric energy density, which could potentially be further improved by charging to high voltage. Practical adoption of the high-voltage charging is, however, hindered by LiCoO 2 's structural instability at the deeply delithiated state and the associated safety concerns. Here, we achieve stable cycling of LiCoO 2 at 4.6 V (vs. Li/Li +) through trace Ti-Mg-Al co-doping. By using state-of-the-art synchrotron X-ray imaging and spectroscopic techniques, we confirm the incorporation of Mg and Al into the LiCoO 2 lattice, which inhibits the undesired phase transition at voltages above 4.5 V. On the other hand, even in trace amount, Ti segregates significantly at grain boundaries and on the surface, modifying the microstructure of the particles while stabilizing the surface oxygen at high voltage. These dopants contribute though different mechanisms and synergistically promote the cycle stability of LiCoO 2 at 4.6 V.

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, Cr3+ and Ti4+. The designed P2-Na0.6[Cr0.6Ti0.4]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-Na0.6[Cr0.6Ti0.4]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.