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ACS Nano. 2015 Aug 25;9(8):8430-9. doi: 10.1021/acsnano.5b03274. Epub 2015 Aug 10.

Structural Defects of Silver Hollandite, Ag(x)Mn8O(y), Nanorods: Dramatic Impact on Electrochemistry.

Author information

1
Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory , Upton, New York 11973, United States.
2
SEU-FEI Nano-Pico Center, Key Laboratory of MEMS of the Ministry of Education, School of Electronic Science and Engineering, Southeast University , Nanjing, 210096, PR China.
3
Department of Materials Science and Engineering, Stony Brook University , Stony Brook, New York 11794, United States.
4
Department of Chemistry, Stony Brook University , Stony Brook, New York 11794, United States.
5
Photon Science Division, National Synchrotron Light Source II, Brookhaven National Laboratory , Upton, New York 11973, United States.
6
Energy Sciences Directorate, Brookhaven National Laboratory , Upton, New York 11973, United States.

Abstract

Hollandites (OMS-2) are an intriguing class of sorbents, catalysts, and energy storage materials with a tunnel structure permitting one-dimensional insertion and deinsertion of ions and small molecules along the c direction. A 7-fold increase in delivered capacity for Li/AgxMn8O16 electrochemical cells (160 versus 23 mAh/g) observed upon a seemingly small change in silver content (x ∼1.1 (L-Ag-OMS-2) and 1.6 (H-Ag-OMS-2)) led us to characterize the structure and defects of the silver hollandite material. Herein, Ag hollandite nanorods are studied through the combined use of local (atomic imaging, electron diffraction, electron energy-loss spectroscopy) and bulk (synchrotron based X-ray diffraction, thermogravimetric analysis) techniques. Selected area diffraction and high resolution transmission electron microscopy show a structure consistent with that refined by XRD; however, the Ag occupancy varies significantly even within neighboring channels. Both local and bulk measurements indicate a greater quantity of oxygen vacancies in L-Ag-OMS-2, resulting in lower average Mn valence relative to H-Ag-OMS-2. Electron energy loss spectroscopy shows a lower Mn oxidation state on the surface relative to the interior of the nanorods, where the average Mn valence is approximately Mn(3.7+) for H-Ag-OMS-2 and Mn(3.5+) for L-Ag-OMS-2 nanorods, respectively. The higher delivered capacity of L-Ag-OMS-2 may be related to more oxygen vacancies compared to H-Ag-OMS-2. Thus, the oxygen vacancies and MnO6 octahedra distortion are assumed to open the MnO6 octahedra walls, facilitating Li diffusion in the ab plane. These results indicate crystallite size and surface defects are significant factors affecting battery performance.

KEYWORDS:

electron energy loss spectroscopy; lithium battery; octahedral molecular sieve; oxygen defects; silver hollandite; transmission electron microscopy

PMID:
26181235
DOI:
10.1021/acsnano.5b03274

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