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 (tau-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 tau-MnO2 nanorods (NRs) mainly composed of specific 4 x 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 Na0.25MnO2 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 tau-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 tau-MnO2 material, with the hope of assistance in designing SIBs with high-rate capability based on homogeneous tunnel-specific phase.