Unveiling the Structural Evolution of Ag_1.2Mn₈O₁₆ under Coulombically Controlled (De)Lithiation

MnO₂ materials are considered promising cathode materials for rechargeable lithium, sodium, and magnesium batteries due to their earth abundance and environmental friendliness. One polymorph of MnO₂, α-MnO₂, has 2 × 2 tunnels (4.6 Å × 4.6 Å) in its structural framework, which provide facile diffusion pathways for guest ions. In this work, a silver-ion-containing α-MnO₂ (Ag_1.2Mn₈O₁₆) is examined as a candidate cathode material for Li based batteries. Electrochemical stability of Ag_1.2Mn₈O₁₆ is investigated through Coulombically controlled reduction under 2 or 4 molar electron equivalents (e.e.). Terminal discharge voltage remains almost constant under 2 e.e. of cycling, whereas it continuously decreases under repetitive reduction by 4 e.e. Thus, detailed structural analyses were utilized to investigate the structural evolution upon lithiation. Significant increases in lattice <i>a</i> (17.7%) and atomic distances (∼4.8%) are observed when <i>x</i> in Li_<i>x</i>Ag_1.2Mn₈O₁₆ is >4. Ag metal forms at this level of lithiation concomitant with a large structural distortion to the Mn–O framework. In contrast, lattice <i>a</i> only expands by 2.2% and Mn–O/Mn-Mn distances show minor changes (∼1.4%) at <i>x</i> < 2. The structural deformation (tunnel breakage) at <i>x</i> > 4 inhibits the recovery of the original structure, leading to poor cycle stability at high lithiation levels. This report establishes the correlation among local structure changes, amorphization processes, formation of Ag⁰, and long-term cycle stability for this silver-containing α-MnO₂ type material at both low and high lithiation levels

Structural Defects of Silver Hollandite, Ag_<i>x</i>Mn₈O_<i>y</i>, Nanorods: Dramatic Impact on Electrochemistry

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 <i>c</i> direction. A 7-fold increase in delivered capacity for Li/Ag_<i>x</i>Mn₈O₁₆ electrochemical cells (160 <i>versus</i> 23 mAh/g) observed upon a seemingly small change in silver content (<i>x</i> ∼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 <i>ab</i> plane. These results indicate crystallite size and surface defects are significant factors affecting battery performance

Deliberately Designed Atomic-Level Silver-Containing Interface Results in Improved Rate Capability and Utilization of Silver Hollandite for Lithium-Ion Storage

α-MnO₂-structured materials are generally classified as semiconductors; thus, we present a strategy to increase electrochemical utilization through the design of a conductive material interface. Surface treatment of silver hollandite (Ag_<i>x</i>Mn₈O₁₆) with Ag⁺ (Ag₂O) provides significant benefits to the resultant electrochemistry, including a decreased charge-transfer resistance and a 2-fold increase in deliverable energy density at a high rate. The improved function of this designed interface relative to conventional electrode fabrication strategies is highlighted

Electrode Reaction Mechanism of Ag₂VO₂PO₄ Cathode

The high capacity of primary lithium-ion cathode Ag₂VO₂PO₄ is facilitated by both displacement and insertion reaction mechanisms. Whether the Ag extrusion (specifically, Ag reduction with Ag metal displaced from the host crystal) and V reduction are sequential or concurrent remains unclear. A microscopic description of the reaction mechanism is required for developing design rules for new multimechanism cathodes, combining both displacement and insertion reactions. However, the amorphization of Ag₂VO₂PO₄ during lithiation makes the investigation of the electrode reaction mechanism difficult with conventional characterization tools. For addressing this issue, a combination of local probes of pair-distribution function and X-ray spectroscopy were used to obtain a description of the discharge reaction. We determine that the initial reaction is dominated by silver extrusion with vanadium playing a supporting role. Once sufficient Ag has been displaced, the residual Ag⁺ in the host can no longer stabilize the host structure and V–O environment (i.e., onset of amorphization). After amorphization, silver extrusion continues but the vanadium reduction dominates the reaction. As a result, the crossover from primarily silver reduction displacement to vanadium reduction is facilitated by the amorphization that makes vanadium reduction increasingly more favorable