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I8.06 - In-situ TEM Study on the Tunnel Evolution during Lithiation of Single Crystalline Alpha-MnO2 Nanowires 
April 9, 2015   3:30pm - 3:45pm

Manganese dioxide (MnO2) is widely known to possess various allotropic forms such as ?-, ?- and ?-phases, which are constructed by combination of octahedral [MnO6] building blocks to form different tunneled structures. These special structures are believed to account for the various characteristics of MnO2 when it is employed as electrode material in lithium (ion) batteries. There is, however, lack of direct proof demonstrating the role of tunneled structure during electrochemical lithiation/delithiation of MnO2. In this work, by applying high resolution scanning transmission electron microscopy (HRSTEM) to single ?-MnO2 nanowire along both axial and radial directions, the tunneled structure is clearly shown and characterized. The ?-MnO2 nanowire is proved to be single crystalline and grow along [001] direction. Cross-sectional HRSTEM images have shown that the nanowire has a squared cross section and 2x2 tunnels align parallelly along its growth direction [001], matching very well with simulated crystal structure. An in-situ TEM setup for study of MnO2�s dynamic lithiation/delithiation process is also designed and demonstrated. It is found that upon lithiation, the ?-MnO2 nanowire shows different orientation-sensitive morphologies. That is, ?-MnO2 unit cell expands asynchronously along [100] and [010] directions, resulting in macroscopic difference under [010] and [100] zone axes observations. DFT simulation demonstrates that such an asynchronous expansion originates from the specific Li-occupancy sequence at Whckoff 8h sites inside ?-MnO2�s 2�2 tunnels.

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