Structural Defects of Silver Hollandite, Ag_xMn₈O_y, Nanorods: Dramatic Impact on Electrochemistry

Journal Article · 16 July 2015 · ACS Nano

DOI: https://doi.org/10.1021/acsnano.5b03274 · OSTI ID:1370520

Wu, Lijun ^[1]; Xu, Feng ^[2]; Zhu, Yimei ^[1]; Brady, Alexander B. ^[3]; Huang, Jianping ^[3]; Durham, Jessica L. ^[3]; Dooryhee, Eric ^[1]; Marschilok, Amy C. ^[3]; Takeuchi, Esther S. ^[4]; Takeuchi, Kenneth J. ^[3]

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1. Brookhaven National Lab. (BNL), Upton, NY (United States)
2. Brookhaven National Lab. (BNL), Upton, NY (United States); Southeast Univ., Nanjing (China)
3. Stony Brook Univ., NY (United States)
4. Stony Brook Univ., NY (United States); Brookhaven National Lab. (BNL), Upton, NY (United States)

Hollandites (OMS-2) are an enticing 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/Ag_xMn₈O₁₆ 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. In this work, 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.

• Research Organization: Energy Frontier Research Centers (EFRC) (United States). Center for Mesoscale Transport Properties (m2M); Brookhaven National Laboratory (BNL), Upton, NY (United States)
• Sponsoring Organization: USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22). Materials Sciences & Engineering Division
• Grant/Contract Number: SC0012673; SC0012704
• OSTI ID: 1370520
• Journal Information: ACS Nano, Vol. 9, Issue 8; Related Information: m2M partners with Stony Brook University (lead); Brookhaven National Laboratory; Columbia University; Georgia Institute of Technology; Oak Ridge National Laboratory; Rensselaer Polytechnic Institute; University of California, Berkeley; University of North Carolina at Chapel Hill; ISSN 1936-0851
• Publisher: American Chemical Society (ACS)Copyright Statement
• Country of Publication: United States
• Language: English