Mechanistic Insight into Transition Metal (Tm = Cu, Fe, and Co) Doping on the Electrochemical Performance of Na0.67Tm0.2Ti0.125Mn0.675O2 Cathodes Involving a Ti(III)/Ti(IV) Redox Couple

Abstract

The layered P2-type oxide Na0.67MnO2 is widely recognized as a promising cathode material for sodium-ion batteries and has attracted considerable research interest. However, as a manganese-based material, it is susceptible to issues such as the Jahn−Teller effect and high-voltage-induced phase transitions, which compromise structural stability during cycling and lead to rapid capacity degradation. Although doping with inert metal ions is commonly adopted to enhance cycling stability, it often results in a reduced initial capacity owing to the electrochemical inactivity of these dopants. Herein, we propose a codoping strategy incorporating multivalent Ti(III)/Ti(IV) and transition metal ions (Tm = Cu, Fe, and Co) as codopants, creating a synergistic redox-active environment that simultaneously enhances initial capacity and cycling stability. The dQ/dV curves reveal a new redox peak in the 1.7−1.9 V range for the prepared codoped P2-type cathode materials, which offsets the capacity loss associated with Mn substituted. In contrast, this feature is absent in Na0.67Ti0.125Mn0.875O2 doped exclusively with Ti(III)/Ti(IV). Ex situ X-ray photoelectron spectroscopy and X-ray diffraction analysis within this voltage window confirm that the redox peak arises from the synergistic interaction between Ti(III)/Ti(IV) redox couples and the migration of Tm ions. Valence changes of the dopant ions during ion migration thus contribute additional capacity and consequently influence the intensity of the new redox peak. Among the materials investigated, the Co ion and Ti(III)/Ti(IV) codoped Na0.67Co0.2Ti0.125Mn0.675O2 exhibits superior electrochemical performance. It delivers an initial capacity of 142 mAh g−1 in half-cells and 97 mAh g−1 in full cells, with capacity retentions of 81% and 73% after 100 cycles, respectively. This study demonstrates that manipulating the oxidation states of dopant ions offers a promising strategy for designing high-performance layered oxide cathodes.