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

Reduction of silver ions in molybdates: elucidation of framework acidity as the factor controlling charge balance mechanisms in aqueous zinc-ion electrolyte

Across four molybdates, reduction of silver ions in aqueous zinc electrolyte is more facile with increasing acidity.

Vanadium-Substituted Tunnel Structured Silver Hollandite (Ag1.2VxMn8-xO16): Impact on Morphology and Electrochemistry

A series of tunnel structured V-substituted silver hollandite (Ag_1.2V_xMn_8-xO₁₆, x = 0-1.4) samples is prepared and characterized through a combination of synchrotron X-ray diffraction (XRD), synchrotron X-ray absorption spectroscopy (XAS), laboratory Raman spectroscopy, and electron microscopy measurements. The oxidation states of the individual transition metals are characterized using V and Mn K-edge XAS data indicating the vanadium centers exist as V⁵⁺, and the Mn oxidation state decreases with increased V substitution to balance the charge. Scanning transmission electron microscopy of reduced materials shows reduction-displacement of silver metal at high levels of lithiation. In lithium batteries, the V-substituted tunneled manganese oxide materials reveal previously unseen reversible nonaqueous Ag electrochemistry and exhibit up to 2.5× higher Li storage capacity relative to their unsubstituted counterparts. The highest capacity was observed for the Ag_1.2(V_0.8Mn_7.2)O₁₆·0.8H₂O material with an intermediate level of V substitution, likely due to a combination of the atomic composition, the morphology of the particle, and the homogeneous distribution of the active material within the electrode structure where factors over multiple length scales contribute to the electrochemistry.

Investigation of α-MnO2 Tunneled Structures as Model Cation Hosts for Energy Storage

Future advances in energy storage systems rely on identification of appropriate target materials and deliberate synthesis of the target materials with control of their physiochemical properties in order to disentangle the contributions of distinct properties to the functional electrochemistry. This goal demands systematic inquiry using model materials that provide the opportunity for significant synthetic versatility and control. Ideally, a material family that enables direct manipulation of characteristics including composition, defects, and crystallite size while remaining within the defined structural framework would be necessary. Accomplishing this through direct synthetic methods is desirable to minimize the complicating effects of secondary processing. The structural motif most frequently used for insertion type electrodes is based on layered type structures where ion diffusion in two dimensions can be envisioned. However, lattice expansion and contraction associated with the ion movement and electron transfer as a result of repeated charge and discharge cycling can result in structural degradation and amorphization with accompanying loss of capacity. In contrast, tunnel type structures embody a more rigid framework where the inherent structural design can accommodate the presence of cations and often multiple cations. Of specific interest are manganese oxides as they can exhibit a tunneled structure, termed α-MnO₂, and are an important class of nanomaterial in the fields of catalysis, adsorption-separation, and ion-exchange. The α-MnO₂ structure has one-dimensional 2 × 2 tunnels formed by corner and edge sharing manganese octahedral [MnO₆] units and can be readily substituted in the central tunnel by a variety of cations of varying size. Importantly, α-MnO₂ materials possess a rich chemistry with significant synthetic versatility allowing deliberate synthetic control of structure, composition, crystallite size, and defect content. This Account considers the investigation of α-MnO₂ tunnel type structures and their electrochemistry. Examination of the reported findings on this material family demonstrates that multiple physiochemical properties influence the electrochemistry. The retention of the parent structure during charge and discharge cycling, the material composition including the identity and content of the central cation, the surface condition including oxygen vacancies, and crystallite size have all been demonstrated to impact electrochemical function. The selection of the α-MnO₂ family of materials as a model system and the ability to control the variables associated with the structural family affirm that full investigation of the mechanisms related to active materials in an electrochemical system demands concerted efforts in synthetic material property control and multimodal characterization, combined with theory and modeling. This then enables more complete understanding of the factors that must be controlled to achieve consistent and desirable outcomes.