Lattice-resolution visualization of anisotropic sodiation degrees and revelation of sodium storage mechanisms in todorokite-type MnO₂ with in-situ TEM

Todorokite-type manganese dioxide (τ-MnO₂) with large tunneled structure has been considered a promising electrode material used in sodium-ion batteries (SIBs) for large-scale energy storage systems. Precise understanding of sodium storage mechanisms in such large tunnels, however, still remains ambiguous due to a lack of direct atomic-level observation. Here, structural evolutions of τ-MnO₂ nanorods (NRs) mainly composed of specific 4 × 3 tunnels during (de)sodiation are studied carefully with in-situ transmission electron microscopy, including lattice-resolution imaging, consecutive electron diffraction, and electron energy loss spectroscopy, coupled with density functional theory calculations. By real-time tracing the full sodiation process, multistep phase conversion reactions are revealed, beginning with tunnel-based Na⁺-intercalation, undergoing the formation of intermediate Na_0.25MnO₂ and NaMnO₂ phases as result of tunnel distortion and degradation, and ending with the final MnO phase. Furthermore, we witness the first lattice-level visualization of different sodiation degrees correlated with crystallography orientations under the same field of view, unveiling the anisotropic contraction and expansion of lattice a and c upon inserting Na⁺ ions, as corroborated by density functional theory (DFT) calculations. During the following desodiation, the extraction of Na⁺ ions causes the recolonization of the NaMnO₂ phase (rather than the original τ-MnO₂). Subsequently, a reversible and symmetric conversion reaction between MnO and NaMnO₂ phases is established upon the repeated (de)sodiation cycles. This work affords valuable insights into electrochemical sodium storage mechanisms of tunnel-structured τ-MnO₂ material, with the hope of assistance in designing SIBs with high-rate capability based on homogeneous tunnel-specific phase.