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.

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.

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.

Electric energy storage systems such as batteries can significantly impact society in a variety of ways, including facilitating the widespread deployment of portable electronic devices, enabling the use of renewable energy generation for local off grid situations and providing the basis of highly efficient power grids integrated with energy production, large stationary batteries, and the excess capacity from electric vehicles. A critical challenge for electric energy storage is understanding the basic science associated with the gap between the usable output of energy storage systems and their theoretical energy contents. The goal of overcoming this inefficiency is to achieve more useful work (w) and minimize the generation of waste heat (q). Minimization of inefficiency can be approached at the macro level, where bulk parameters are identified and manipulated, with optimization as an ultimate goal. However, such a strategy may not provide insight toward the complexities of electric energy storage, especially the inherent heterogeneity of ion and electron flux contributing to the local resistances at numerous interfaces found at several scale lengths within a battery. Thus, the ability to predict and ultimately tune these complex systems to specific applications, both current and future, demands not just parametrization at the bulk scale but rather specific experimentation and understanding over multiple length scales within the same battery system, from the molecular scale to the mesoscale. Herein, we provide a case study examining the insights and implications from multiscale investigations of a prospective battery material, Fe3O4.

This review highlights the efficacy of EDXRD as a non-destructive characterization tool in elucidating system-level phenomena for batteries.