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-Na_0.643 Li_0.078 Mn_0.827 Ti_0.095 O₂ is synthesized through the incorporation of Li and Ti into the distorted orthorhombic P'2-Na_0.67 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 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 ).

Keywords: cycle stability; electron population; layered oxides; sodium-ion batteries; solid-solution reaction.