4.8 Article

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

期刊

ENERGY STORAGE MATERIALS
卷 37, 期 -, 页码 345-353

出版社

ELSEVIER
DOI: 10.1016/j.ensm.2021.02.023

关键词

Todorokite-type MnO2; Lattice-resolution; In-situ transmission electron microscopy; Sodium storage mechanism; Crystallographic orientation-dependent sodiation degree

资金

  1. National Key Research & Development Program of China [2020YFB2007400]
  2. National Natural Science Foundation of China [51972058, 11774051, 61574034, 51372039, 21243011, 61674029, 11902284]
  3. National Basic Research Program of China (973 Program) [2015CB352106]
  4. Scientific Research Foundation of Graduate School of Southeast University [YBPY2026]

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This study investigates the structural evolution of tau-MnO2 nanorods during sodiation using in-situ transmission electron microscopy, revealing multistep phase conversion reactions and lattice-level visualization of different sodiation degrees. Anisotropic contraction and expansion of lattice a and c upon inserting Na+ ions are observed, providing valuable insights into electrochemical sodium storage mechanisms in tunnel-structured tau-MnO2 material.
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.

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