Synthetic Control of Composition and Crystallite Size of Silver Hollandite, Ag_xMn₈O₁₆: Impact on Electrochemistry

Abstract

Synthetic control of the silver content in silver hollandite, AgxMn8O16, where the silver content ranges from 1.0 ≤ × ≤ 1.8 is demonstrated. This level of compositional control was enabled by the development of a lower temperature reflux based synthesis compared to the more commonly reported hydrothermal approach. Notably, the synthetic variance of the silver content was accompanied by a concomitant variance in crystallite size as well as surface area and particle size. In order to verify the retention of the hollandite structure, the first Rietveld analysis of silver hollandite was conducted on samples of varying composition. The impacts of silver content, crystallite size, surface area, and particle size on electrochemical reversibility were examined under cyclic voltammetry and battery testing.

Structural Defects of Silver Hollandite, AgxMn8Oy, Nanorods: Dramatic Impact on Electrochemistry

“…13 Subsequent studies have used powder diffraction data and relied on Jansen's structure. 18 In keeping with previous literature, we have also used Jansen's I4/m structure. The Rietveld refinement of the silver hollandite nanorod powders is difficult due to significant crystal shape anisotropy of the samples.…”

“…More recently, we reported a reflux preparation including the ability to tune the material properties of silver hollandite (Ag x Mn 8 O 16 ) via modifications in the reflux synthesis conditions. 17,18 With a change in Ag to Mn ratios, we observed a concomitant variance in crystallite size and associated particle size and bulk surface area, demonstrating ability to synthetically control the material properties.…”

“…17 Further, the synthetic control enabling systematic variation of physical properties facilitated a more detailed exploration of the electrochemical behavior of silver hollandite. 18 Electrochemical testing was conducted for a series of silver hollandite, Ag x Mn 8 O 16 3 nH 2 O, samples over a range of silver (x) content, ranging from 1.0 e x e 1.8, n ∼ 2. Constant current discharge, pulse testing, galvanostatic intermittent titration type (GITT) testing and cycle testing each demonstrated improved performance of the smaller crystallite size materials with improved reversibility and delivered capacity as a result of crystallite size/silver content reduction.…”

The mineralogical characterization of argentian cryptomelane from Xiangguang Mn–Ag deposit, North China

“…Recently manganese oxides with the cryptomelane structure modified with silver have been demonstrated as Ag-hollandite (Ag x Mn 8 O 16 , where 1 ≤ x ≤ 1.8) by many material scientists. The Ag + center in a tunnel consisting of double chains of edge-sharing MnO 6 octahedra has the potential to be electrochemically and catalytically active (Takeuchi et al, 2012(Takeuchi et al, , 2013Özacar et al, 2013). The presence of silver causes partial replacement of potassium from the channels and distortion of the cryptomelane structure (Gac, 2006 (Takeuchi et al, 2012).…”

Synthesis of Copper Birnessite, CuxMnOy·nH2O with Crystallite Size Control: Impact of Crystallite Size on Electrochemistry

“…Manganese-based materials are environmentally friendly and inexpensive when compared to materials based on cobalt or nickel, and thus have received much attention in the battery research literature. For example, several types of manganese oxide materials including hollandite (2 × 2 tunnel, OMS-2), [4][5][6][7] and todorokite (3 × 3 tunnel, OMS-3) 8 have been investigated for battery applications, where quasi-reversible lithium-ion electrochemistry in reported.…”