Developing scalable energy storage systems with high energy and power densities is essential to meeting the ever-growing portable electronics and electric vehicle markets, which calls for development ...of thick electrode designs to improve the active material loading and greatly enhance the overall energy density. However, rate capabilities in lithium-ion batteries usually fall off rapidly with increasing electrode thickness due to hindered ionic transport kinetics, which is especially the issue for conversion-based electroactive materials. To alleviate the transport constrains, rational design of three-dimensional porous electrodes with aligned channels is critically needed. Herein, magnetite (Fe3O4) with high theoretical capacity is employed as a model material, and with the assistance of micrometer-sized graphine oxide (GO) sheets, aligned Fe3O4/GO (AGF) electrodes with well-defined ionic transport channels are formed through a facile ice-templating method. The as-fabricated AGF electrodes exhibit excellent rate capacity compared with conventional slurry-casted electrodes with an areal capacity of ∼3.6 mAh·cm–2 under 10 mA·cm–2. Furthermore, clear evidence provided by galvanostatic charge–discharge profiles, cyclic voltammetry, and symmetric cell electrochemical impedance spectroscopy confirms the facile ionic transport kinetics in this proposed design.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Electrochemical energy storage systems, specifically lithium and lithium-ion batteries, are ubiquitous in contemporary society with the widespread deployment of portable electronic devices. Emerging ...storage applications such as integration of renewable energy generation and expanded adoption of electric vehicles present an array of functional demands. Critical to battery function are electron and ion transport as they determine the energy output of the battery under application conditions and what portion of the total energy contained in the battery can be utilized. This review considers electron and ion transport processes for active materials as well as positive and negative composite electrodes. Length and time scales over many orders of magnitude are relevant ranging from atomic arrangements of materials and short times for electron conduction to large format batteries and many years of operation. Characterization over this diversity of scales demands multiple methods to obtain a complete view of the transport processes involved. In addition, we offer a perspective on strategies for enabling rational design of electrodes, the role of continuum modeling, and the fundamental science needed for continued advancement of electrochemical energy storage systems with improved energy density, power, and lifetime.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
2D nanosheets have been widely explored as electrode materials owing to their extraordinarily high electrochemical activity and fast solid‐state diffusion. However, the scalable electrode fabrication ...based on this type of material usually suffers from severe performance losses due to restricted ion‐transport kinetics in a large thickness. Here, a novel strategy based on evaporation‐induced assembly to enable directional ion transport via forming vertically aligned nanosheets is reported. The orientational ordering is achieved by a rapid evaporation of mixed solvents during the electrode fabrication process. Compared with conventional drop‐cast electrodes, which exhibit a random arrangement of the nanosheets and obvious decrease of rate performance with increasing thickness, the electrode based on the vertically aligned nanosheets is able to retain the original high rate capability even at high mass loadings and electrode thickness. Combined electrochemical and structural characterization reveals the electrode composed of orientation‐controlled nanosheets to possess lower charge‐transfer resistances, leading to more complete phase transformation in the active material.
Fabrication of electrodes based on dense and vertically aligned nanosheets is achieved via a controlled solvent‐evaporation process. The structurally advantageous architecture features excellent rate capabilities, lower charge‐transfer resistances, and more complete phase transformation of active material, much superior to the arrangement of stacked nanosheets commonly found using the conventional electrode preparation method.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Rechargeable aqueous Zn/α-MnO2 batteries are a possible alternative to lithium ion batteries for scalable stationary energy storage applications due to their low cost, safety and environmentally ...benign components. A critical need for advancement of this battery system is a full understanding of the electrochemical reaction mechanisms, which remain unclear. In this report, operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry. Simultaneous visualization and quantification of the Mn distribution in the electrolyte revealed the formation of aqueous Mn species during discharge and depletion on charge. The findings are supported by ex situ transmission electron microscopy (TEM), X-ray diffraction, Mn K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) measurements. The elucidated mechanism is fundamentally different from the previously proposed Zn2+ insertion or conversion reactions. These findings provide a foundation for developing dissolution- deposition chemistries suitable for scalable stationary energy storage with aqueous electrolyte.
Zn/MnO2 systems using non-alkaline aqueous electrolytes have attracted tremendous interest as rechargeable aqueous Zn ion batteries due to their safety and high specific capacities. Despite their ...promising electrochemical performance, however, their reaction mechanism has remained elusive. Here, we examined the structural evolution of cryptomelane α-MnO2 cathode by ex situ transmission electron microscopy after electrochemical testing using two different non-alkaline aqueous Zn electrolytes with acetate and triflate salts of different pH values. We have discovered that the systems tested in both electrolytes exhibit a dissolution–deposition reaction mechanism through dissolution–deposition of Mn2+ ions from/on the cathodes with a display of similar discharge/charge product formation. We have also found that the cell tested using the acetate electrolyte shows evidence of structural irreversibility that might contribute to its rapid capacity degradation. This finding offers an important insight into optimizing the cathode design for enhanced electrochemical function of aqueous Zn/MnO2 batteries.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Sodium vanadate (Na1+x V3O8 or NVO) has recently attracted significant interest as a potential cathode material for an aqueous Zn ion battery for its unique pillared framework facilitating Zn ion ...migration. Here, we performed a detailed study on reaction mechanisms of hydrated Na2V6O16·2H2O slabs and nonhydrated Na1.25V3O8 nanorods using transmission electron microscopy. Our initial observation reveals that the thin (30–50 nm) Na2V6O16·2H2O system successfully undergoes discharge with Zn ion insertion into the structure while thick (120–170 nm) Na1.25V3O8 allows Zn ion insertion only at the surface, signifying the importance of both the presence of water and the nanostructure thickness in determining the reaction mechanism of NVO. More in-depth analysis of these two systems revealed the irreversible formation of the stable byproduct phase Zn3Na x (OH2)V2O7 (ZNVO), which likely evolved through a Zn-ion redox reaction, contributing to overall cell performance. Eventually, the entire discharge/charge process appears to become bifurcated, consisting of primary (Zn redox in NVO) and secondary (Zn redox in ZNVO) reactions, where their relative contribution to overall cell capacity changes with continued cycling. Our study provides a fresh insight into the morphology- and hydration-dependent reaction mechanisms of NVO and their implications on the electrochemistry.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Hollandite, α-MnO2, is of interest as a prospective cathode material for hydrated zinc-ion batteries (ZIBs); however, the mechanistic understanding of the discharge process remains limited. Herein, a ...systematic study on the initial discharge of an α-MnO2 cathode under a hydrated environment was reported using density functional theory (DFT) in combination with complementary experiments, where the DFT predictions well described the experimental measurements on discharge voltages and manganese oxidation states. According to the DFT calculations, both protons (H+) and zinc ions (Zn2+) contribute to the discharging potentials of α-MnO2 observed experimentally, where the presence of water plays an essential role during the process. This study provides valuable insights into the mechanistic understanding of the discharge of α-MnO2 in hydrated ZIBs, emphasizing the crucial interplay among the H2O molecules, the intercalated Zn2+ or H+ ions, and the Mn4+ ions on the tunnel wall to enhance the stability of discharged states and, thus, the electrochemical performances in hydrated ZIBs.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Aqueous Zn/MnO 2 batteries with mildly acidic electrolytes are promising candidates for low cost, high safety electrochemical energy storage for grid-scale applications. However, the complexity of ...the chemistry results in conflicting reports of operation principles, making rational improvements challenging. In this work, operando synchrotron X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) as well as ex situ Raman spectroscopy, XRD, and XAS characterization were used to probe the mechanism of aqueous Zn/α-MnO 2 batteries with ZnSO 4 electrolyte. A multi-stage Mn dissolution–conversion charge storage mechanism was revealed, which consists of reversible solid-aqueous phase transformation via Mn dissolution–deposition reactions and a solid redox mechanism via Zn-ion insertion. This mechanism is supported by thermodynamic calculations paired with in situ electrolyte pH measurements to provide further mechanistic insights. The findings establish a detailed charge storage mechanism for aqueous Zn/α-MnO 2 batteries with a well resolved reversible layered charge product structure, that can serve as a reference for future studies on advancing the reversibility and stability of aqueous Zn/α-MnO 2 batteries.
Aqueous Zn/MnO2 batteries (AZMOB) with mildly acidic electrolytes hold promise as potential green grid-level energy storage solutions for clean power generation. Mechanistic understanding is critical ...to advance capacity retention needed by the application but is complex due to the evolution of the cathode solid phases and the presence of dissolved manganese in the electrolyte due to a dissolution–deposition redox process. This work introduces operando multiphase extended X-ray absorption fine structure (EXAFS) analysis enabling simultaneous characterization of both aqueous and solid phases involved in the Mn redox reactions. The methodology was successfully conducted in multiple electrolytes (ZnSO4, Zn(CF3SO3)2, and Zn(CH3COO)2) revealing similar manganese coordination environments but quantitative differences in distribution of Mnn+ species in the solid and solution phases. Complementary Raman spectroscopy was utilized to identify the less crystalline Mn-containing products formed under charge at the cathodes. This was further augmented by transmission electron microscopy (TEM) to reveal the morphology and surface condition of the deposited solids. The results demonstrate an effective approach for bulk-level characterization of poorly crystalline multiphase solids while simultaneously gaining insight into the dissolved transition-metal species in solution. This work provides demonstration of a useful approach toward gaining insight into complex electrochemical mechanisms where both solid state and dissolved active materials are important contributors to redox activity.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
Conspectus 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.
Full text
Available for:
IJS, KILJ, NUK, PNG, UL, UM
PDF
