Direct Observation of an Anomalous Spinel-to-Layered Phase Transition Mediated by Crystal Water Intercalation

The phase transition of layered manganese oxides to spinel phases is a well‐known phenomenon in rechargeable batteries and is the main origin of the capacity fading in these materials. This spontaneous phase transition is associated with the intrinsic properties of manganese, such as its size, preferred crystal positions, and reaction characteristics, and it is therefore very difficult to avoid. The introduction of crystal water by an electrochemical process enables the inverse phase transition from spinel to a layered Birnessite structure. Scanning transmission electron microscopy can be used to directly visualize the rearrangement of lattice atoms, the simultaneous insertion of crystal water, the formation of a transient structure at the phase boundary, and layer‐by‐layer progression of the phase transition from the edge. This research indicates that crystal water intercalation can reverse phase transformation with thermodynamically favored directionality. Reversing a phase transition with water: An anomalous spinel‐to‐layered phase transition is tracked on the atomic scale using scanning transmission electron microscopy. The phase transition is mediated by formation of a transition phase at the phase boundary that traps crystal water in a stepwise manner.