Layered-to-Tunnel Transformation Enabled by Coordination Bond Reorganization in Oxide Cathodes for Sodium–Ion Batteries

Designing composite structures to enhance the performance of manganese-based sodium-ion battery (SIB) cathodes now represents a significant research focus. However, the development of composite-phase cathodes is hindered by an incomplete understanding of their structural transformation mechanisms. Herein, we elucidate the relationship between the transition metal bond energy and material structure, proposing a coordination bond reorganization strategy to drive structural transformations. This concept is demonstrated using the cathode material Na_0.63Mn_1-<i>x</i>Ti_<i>x</i>O₂, showing that the bond energy of transition metals plays a critical role in the structural transformation process. By introducing stronger Ti-O covalent bonds, we achieved the transition from a layered to tunnel structure. Furthermore, the layered-tunnel intergrowth structure Na_0.63Mn_0.95Ti_0.05O₂ demonstrates a remarkable specific capacity of 194.68 mAh g^-1 alongside excellent stability while exhibiting good compatibility with hard carbon anodes. This work validates the feasibility of coordination bond reorganization as a mechanism for autonomous structural transformations, offering new insights for the precise design of high-energy-density materials for next-generation SIBs.