Whole-Voltage-Range Solid-Solution Reaction in Layered Oxide Cathode of Sodium-Ion Batteries.

Abstract

Layered manganese-based oxides (LMOs) are promising cathode materials for sodium-ion batteries (SIBs) due to their versatile structures. However, the Jahn-Teller effect of Mn3+ induces severe distortion of MnO6 octahedra, and the resultant low symmetry is responsible for the gliding of MnO2 layers and then inferior multiple-phase transitions upon Na+ extraction/insertion. Here, hexagonal P2-Na0.643 Li0.078 Mn0.827 Ti0.095 O2 is synthesized through the incorporation of Li and Ti into the distorted orthorhombic P'2-Na0.67 MnO2 to function as a phase-transition-free oxide cathode. It is revealed that Li in both the transition-metal and Na layers enhances the covalency of Mn-O bonds and allows degeneracy of Mn 3d eg orbitals to favor the formation of hexagonal phase, and the high strength of Ti-O bonds reduces the electrostatic interaction between Na and O for suppressed Na+ /vacancy rearrangements. These collectively lead to a whole-voltage-range solid-solution reaction between 1.8 and 4.3V with a small volume variation of 1.49%. This rewards its excellent cycling stability (capacity retention of 90% after 500 cycles) and rate capability (89 mAh g-1 at 2000mA g-1 ).