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.

Operando, spatiotemporal resolved synchrotron X-ray fluorescence mapping measurements on a custom aqueous Zn/α-MnO2 cell provided direct, quantitative evidence of a Mn dissolution-deposition faradaic mechanism that governs the electrochemistry.

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.

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-MnO, and are an important class of nanomaterial in the fields of catalysis, adsorption-separation, and ion-exchange. The α-MnO structure has one-dimensional 2 × 2 tunnels formed by corner and edge sharing manganese octahedral [MnO] units and can be readily substituted in the central tunnel by a variety of cations of varying size. Importantly, α-MnO 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-MnO 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-MnO 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.

A carbon nanotube (CNT) web electrode comprising magnetite spheres and few-walled carbon nanotubes (FWNTs) linked by the carboxylated conjugated polymer, poly[3-(potassium-4-butanoate) thiophene] (PPBT), was designed to demonstrate benefits derived from the rational consideration of electron/ion transport coupled with the surface chemistry of the electrode materials components. To maximize transport properties, the approach introduces monodispersed spherical FeO (sFeO) for uniform Li diffusion and a FWNT web electrode frame that affords characteristics of long-ranged electronic pathways and porous networks. The sFeO particles were used as a model high-capacity energy active material, owing to their well-defined chemistry with surface hydroxyl (-OH) functionalities that provide for facile detection of molecular interactions. PPBT, having a π-conjugated backbone and alkyl side chains substituted with carboxylate moieties, interacted with the FWNT π-electron-rich and hydroxylated sFeO surfaces, which enabled the formation of effective electrical bridges between the respective components, contributing to efficient electron transport and electrode stability. To further induce interactions between PPBT and the metal hydroxide surface, polyethylene glycol was coated onto the sFeO particles, allowing for facile materials dispersion and connectivity. Additionally, the introduction of carbon particles into the web electrode minimized sFeO aggregation and afforded more porous FWNT networks. As a consequence, the design of composite electrodes with rigorous consideration of specific molecular interactions induced by the surface chemistries favorably influenced electrochemical kinetics and electrode resistance, which afforded high-performance electrodes for battery applications.

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.

We report on the design of a battery electrode architecture in which ion and electronic transport pathways are contiguous and span the entire volume of a thick, nonplanar electrode. It is shown that for a range of active materials conductivities, the length scale for electronic transport in such an architecture can be tuned by simple manipulations of the electrode design to enable good access to the active material. The benefits of such electrodes for basic science research and practical lithium metal batteries are demonstrated in low-N:P ratio cells in which a conventional (300−800 μm) Li foil is successfully cycled with LiCoO 2 cathodes with high areal capacities (10−28 mAh/cm 2 ).

Molybdenum(IV) sulfide (MoS 2 ) has generated significant interest as an electroactive material for Li-ion batteries because of its high theoretical capacity, good rate capability, and minimal volume changes during cycling. An important challenge toward implementing this material is understanding the many polymorphs of MoS 2 that can be (de)stabilized by electrochemical lithiation and nanosizing. To this end, bulk MoS 2 and nanosheet-type MoS 2 were characterized both as solids (X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasmaoptical emission spectroscopy (ICP-OES)) and during electrochemical cycling within operando X-ray analysis compatible lithium cells (operando XRD and ex situ XAS). In situ XRD shows that the bulk 2H-MoS 2 phase is converted to 1T-Li x MoS 2 upon discharge and that this change is only partially reversible upon charge. Furthermore, operando XRD identifies the nanosheet MoS 2 as the metastable 1T′ phase and shows that this phase is conserved upon discharge. Ex situ XAS provides additional structural insights into the local structure of MoS 2 , confirming that the 1T′ phase is the correct assignment of the nanosheet MoS 2 and revealing an irreversible local distortion that occurs during cycling. This local distortion is likely a factor in the increased capacity fade observed in the nanosheet cells. This work provides important insights into the structure of MoS 2 and how that structure is affected by nanosizing and cycling, which can inform other studies of nanosheet layered materials.