Energy-Transfer-Modulated Structural Evolution during Lithium-Sodium Ion Exchange in Layered Oxide Cathodes

Ion exchange provides a versatile route to access metastable layered oxide materials beyond conventional thermodynamic limits, yet its development has been constrained by an insufficient understanding of how synthesis pathways govern exchange kinetics, structural evolution, and electrochemical performance. Using Na_0.6Li_0.2Mn_0.8O₂─a P2-type cathode for sodium-ion batteries─as a well-defined model, we uncover how distinct ion-exchange methods─solid-state ball milling and liquid-phase ultrasonication─induce fundamentally different exchange behaviors via distinct energy-transfer modes. Ball milling drives rapid defect-mediated exchange and a stress-activated 1/5 → 1/3 superstructure transition. In contrast, ultrasonication leads to kinetically limited exchange with intralayer disorder through a collective phonon-like mechanism. Atomic-scale imaging reveals that these contrasting modes give rise to distinct interlayer slip dynamics: short-range stress-driven slip in ball-milled samples and long-range cooperative slip under ultrasonication, both propagating layerwise along aligned ion-diffusion channels. Guided by these mechanistic insights, we develop a sequential ball milling-ultrasonication process that achieves near-complete exchange (98.3 %) within 2 h while retaining the structural integrity. Subsequent postannealing repairs defects and yields a cathode with a reversible capacity of 235 mAh/g (versus lithium metal). This work establishes a rational design framework for efficient, structure-preserving cathode synthesis and reveals general principles governing ion-exchange chemistry in solid oxides.