Atomic-level tunnel engineering of todorokite MnO2 for precise evaluation of lithium storage mechanisms by in situ transmission electron microscopy

Todorokite-type manganese oxide (τ-MnO2) with p × 3 tunneled structure is especially captivating as charge storage material for rechargeable batteries, since the enlarged tunnel dimensions enable rapid electrode kinetics and superior rate performance. However, its congenitally rich polytypism associated with tunnel heterogeneity impedes our precise understanding of structure-property relationship in this polytypic material. In this regard, this work has taken substantial effort to preliminarily achieve uniform 4 × 3 tunnel-structured τ-MnO2 nanorods, as corroborated via atomically resolved imaging. Afterwards, the (de)lithiation mechanisms of the tunnel-specific phase are investigated via in situ transmission electron microscopy including electron diffraction, high-resolution imaging, and electron energy loss spectroscopy, coupled with density functional theory calculations and phase field simulations. Upon initial lithiation, the intercalation reaction region β (less than 1.43 Li insertion) is observed as result of rapid lithium-ion diffusion through the tunnels with slightly increased lattice constants. By tracing the full lithiation procedure, the evolution of intermediate Mn2O3 phase and the development of final Mn and Li2O phases are identified in the conversion reaction region α (more than 1.43 Li insertion). These results indicate that τ-MnO2 can be applied to a cathode by intercalation reaction and to an anode by conversion reaction in corresponding to voltage ranges in a lithium-based battery. Upon delithiation, we observe an unusual reciprocating-motion reaction front (different from one-way lithiation reaction front), for which the driven dynamics are delineated based on a phase field model. Impressively, a reversible and symmetric conversion reaction between Mn2O3 phase and Mn + Li2O phases is established upon subsequent (de)lithiation cycles. This work can be regarded as a stepping-stone arousing the appetite of a comprehensive understanding of the highly polytypic material with other tunnel-specific phases.