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

Abstract

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 Mn 3+ induces severe distortion of MnO 6 octahedra, and the resultant low symmetry is responsible for the gliding of MnO 2 layers and then inferior multiple‐phase transitions upon Na + extraction/insertion. Here, hexagonal P2‐Na 0.643 Li 0.078 Mn 0.827 Ti 0.095 O 2 is synthesized through the incorporation of Li and Ti into the distorted orthorhombic P'2‐Na 0.67 MnO 2 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 e g 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 4.3 V 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 2000 mA g −1 ).